add checkConnections service

[afrokick]

• fix tests
• add new feature 'broken connections checker'
• new alive_timeout parameter

I set alive_timeout to 60sec because currently PeerJS client send ping message every 20sec.

So, this feature works only with PeerJS 1.0.0+ (with ping function)

I'm going to add pingInterval option to PeerJS config.

How it works

1. Server save last ping timestamp to Client#lastPing
2. a new service checkBrokenConnections check every 300ms that nowTime - lastPing > alive_timeout. When it true - client disconnected from the server.

[afrokick]

It should fix #4 and #89