Initial websocket support

Can be tested here http://www.websocket.org/echo.html. Just put in `<host>:<PORT>/ws/checks/{check_id}/measurements` and hit connect. 

Not sure how much logging there should be, but logs websocket connect/disconnect and error states.

Added godep for gorilla/websocket
Added README snippet
Added handling for creating/tearing down broadcast websockets based on checkID.
Logging with remote address