Add join receiver callback

[sriramsv]

I know the current implementation of this library only allows the script to send messages to other join clients, it will be very useful if you could add the server which can listen to incoming join messages so that we can send join commands to some remote server through tasker or nodered.

Hope you could add this feature
I will try to send a PR if I manage to get this working in my spare time

[sriramsv]

Added PR for this #9

[nkgilley]

Merged #10 and pushed a new release on pypi, 0.0.7. thanks!