Added Support for Capistrano v3

See #46
Fixing file path
Making proper use of fetch
Using deploy:failed hook
Removed todo
Updated README
Updated README
Removed unnecessary default value check for hipchat_announce
Add capistrano 2 backwards support
Check deploy:failed definition for Capistrano 3 pre 3.1 release

'deploy:failed' is only pre-defined in Capistrano 3.1.
Merge branch 'master' of https://github.com/stillmotion/hipchat-rb into stillmotion-master