prevent label clashes in Django 1.7

fixes #14
use our exception catching mechanisms instead of using celery's exception

celery doesn't capture local vars, but we do, so that's nice.
added support for encoding bytes in our JSONEncoder, fixes #13
made json bytes encoding test work in both python2 and python3
added slack/travis integration

the travis cli also took the liberty to re-indent the whole file...
let's try this with a different key
Merge branch 'feature/fix-celery-locals'
Merge branch 'feature/travis-slack-notifications'
bumped version to 1.3.1
PEP8
Merge branch 'master' of github.com:opbeat/opbeat_python into stacktraces_deepest_last
Cleanups.
1.3.2.