Add support for external sensors

[sque]

It would be very usefull to have external information available like cpu usage locally and remote (ssh?).
A sensor is a way to monitor a resource usage locally or remotely. In general an abstraction can be used that all sensors are shell based (top, iotop, ps etc) and there are is an abstract way of refereeing to shells.

Possible configuration:

[router.foo]
shell=ssh
ssh.remote=router.foobar.local
ssh.user=root
#if pass is omitted, it could interactively ask for password?
# or ssh.key=pkey could also be used
# or ssh.pass for saving password

[fooroutercpu]
shell=router.foo
sensor=cpu,memory,network
network.interfaces = eth0
cpu.method=top

[localusage]
shell=local
sensor=cpu
cpu.method=ps
cpu.process_name=python nsts.py