Update twitter pairing notification content

PT story: https://www.pivotaltracker.com/story/show/83788550

This one is pretty simple. We examine the category of an event instance and modify the Twitter notification based on the value.

There was  request made by Thomas to somehow throttle the number of tweets however I feel that belongs in a separate story and should be implemented as a YAML setting.

edit#1: I see there is one failing unrelated cucumber test... I will git blame and send the dev responsible a request to fix. The other failing cucumber spec... looks like it's my fault. Will fix.

edit#2: failing event_instance cucumber spec fixed
            and apparently the other cucumber failure is intermittent...
the after block is needed because the tweet is triggered by an after_save hook in the event instance model.

we do have tests for the content of tweet, where it is important. in a pair programming tweet we are testing for the presence of the project title. i feel than anything else in the tweet is just content and not really worth testing.