Refactor message formatting out of hipchat.py and campfire.py.
Add a service for slack.com. Requires the same params as webhooks.py.
Fix webhook import in slack.py.
Merging Slack integration from https://github.com/idibon/sprint.ly-services.git
Fixing tests, moving _clean_mentions into MessageServiceBase, adding 'self' to message method
Adding a test for _clean_mentions
Adding Slack tests, adding _request method to webhook service class
Convert MessageServiceBase into static class, add dynamic dispatch.
Merge pull request #15 from sprintly/dynamic-dispatch

Convert MessageServiceBase into static class, add dynamic dispatch.
Merge pull request #13 from sprintly/idibon-slack-cleanup

Slack Integration