also send heartbeats if websockets are used

fixes #14
cancel heartbeat timers on connectionLost in the websocket implementation
Propagate connectionLost signal to wrapped protocols

As specified by @rbann
(https://github.com/DesertBus/sockjs-twisted/pull/15#issuecomment-39563167)
Forward connectionLost in all cases
Merge pull request #27 from INTERACT-IV/pr-15-websocket-heartbeats

PR #15 + connectionLost signal