Implement json/gob interfaces

[lifubang]

close #306

After this patch, we can use json.Marshal, json.UnMarshal, GobEncode, and GobDecode for a Variant variable.

[guelfey]

I'm not 100% convinced of this for multiple reasons:

1. Variant is only really meant for communication over dbus. Directly storing it or persisting it doesn't seem like a good idea to me; you can still handle that case if really needed using conversions in application code.
2. It shouldn't be necessary to use maps for representation - String() already returns a string that can be unambiguously parsed back to the same value (which it does by explicitly including the signature if needed).