Initial websocket support

Added godep for gorilla/websocket
Added README snippet
Added handling for creating/tearing down broadcast websockets based on checkID.

Websocket connections can now be made at
<host>:<PORT>/ws/checks/{check_id}/measurements

Logging with remote address
Put websocket handling behind channels

So there can be concurrent access to sending websocket information.
Initially tried a rwMutex but this proved too complicated for minor gain.
Merge pull request #57 from joekarl/ws-support

Initial websocket support