Add initial implementation of presence channel

Maybe some refactoring is needed to reorganize subscription module
To be continued...
Make member_removed event fire up on ws_terminate
Merge branch 'master' of github.com:edgurgel/pusherl into presence-channels

Conflicts:
	src/pusher_event.erl
Update README.md
Merge pull request #1 from edgurgel/presence-channels

Presence channels