aiomcache not support chinese bytes as a key

[dongweiming]

I want use chinese bytes as a key:

In [1]: import asyncio 
   ...: import aiomcache 
   ...:  
   ...: loop = asyncio.get_event_loop()                                                                                                                                                                                                                                             

In [2]: async def hello_aiomcache(): 
   ...:     mc = aiomcache.Client('127.0.0.1', 11211, loop=loop) 
   ...:     await mc.set(bytes('中文', 'utf-8'), b'123') 
   ...:                                                                                                            

In [3]: loop.run_until_complete(hello_aiomcache())                                                                 
---------------------------------------------------------------------------
ValidationException                       Traceback (most recent call last)
<ipython-input-5-adb51212a606> in <module>
----> 1 loop.run_until_complete(hello_aiomcache())

/usr/local/Cellar/python3/3.6.4_2/Frameworks/Python.framework/Versions/3.6/lib/python3.6/asyncio/base_events.py in run_until_complete(self, future)
    465             raise RuntimeError('Event loop stopped before Future completed.')
    466 
--> 467         return future.result()
    468 
    469     def stop(self):

<ipython-input-4-d36e59b6000e> in hello_aiomcache()
      1 async def hello_aiomcache():
      2     mc = aiomcache.Client('127.0.0.1', 11211, loop=loop)
----> 3     await mc.set(bytes('中文', 'utf-8'), b'123')
      4 

~/.virtualenvs/lyanna-tp9VnnDu/lib/python3.6/site-packages/aiomcache/client.py in wrapper(self, *args, **kwargs)
     18         conn = yield from self._pool.acquire()
     19         try:
---> 20             return (yield from func(self, conn, *args, **kwargs))
     21         except Exception as exc:
     22             conn[0].set_exception(exc)

~/.virtualenvs/lyanna-tp9VnnDu/lib/python3.6/site-packages/aiomcache/client.py in set(self, conn, key, value, exptime)
    250         flags = 0  # TODO: fix when exception removed
    251         resp = yield from self._storage_command(
--> 252             conn, b'set', key, value, flags, exptime)
    253         return resp
    254 

~/.virtualenvs/lyanna-tp9VnnDu/lib/python3.6/site-packages/aiomcache/client.py in _storage_command(self, conn, command, key, value, flags, exptime, cas)
    218         # however custom-compiled memcached can have different limit
    219         # so, we'll let the server decide what's too much
--> 220         assert self._validate_key(key)
    221 
    222         if not isinstance(exptime, int):

~/.virtualenvs/lyanna-tp9VnnDu/lib/python3.6/site-packages/aiomcache/client.py in _validate_key(self, key)
     54                 raise ValidationException('trailing newline', key)
     55         else:
---> 56             raise ValidationException('invalid key', key)
     57 
     58         return key

ValidationException: invalid key: b'\xe4\xb8\xad\xe6\x96\x87'

In [4]: debug                                                                                                      
> /Users/dongweiming/.virtualenvs/lyanna-tp9VnnDu/lib/python3.6/site-packages/aiomcache/client.py(56)_validate_key()
     54                 raise ValidationException('trailing newline', key)
     55         else:
---> 56             raise ValidationException('invalid key', key)
     57 
     58         return key

ipdb> key                                                                                                          
b'\xe4\xb8\xad\xe6\x96\x87'

[dongweiming]

ref: https://github.com/aio-libs/aiomcache/blob/master/aiomcache/client.py#L42

Could you let _valid_key_re cover a larger chars?