Refactoring of FollowerCount and Creation of the Videos Table to Support the YouTube Plugin

Hi,

I've fixed the Export Controller test failures. I can't replicate the one occurrence of the exception in testGetAllQuestionPosts()

https://travis-ci.org/ginatrapani/ThinkUp/jobs/9480361

I'm thinking it might just be a transient failure as it never occurred in the other two test runs.

https://travis-ci.org/ginatrapani/ThinkUp/builds/9480360

These two commits change the follower_count table to count_history and create the videos table and supporting classes.

They are part of the YouTube plugin but if we can get them merged now whilst the plugin is undergoing testing it will make testing the YouTube related insights easier.

Thanks

Aaron