Use categories in recommendations, remove self.

We aren't really using tags anymore, so we should use categories to
generate the recommended badges. Also, the badge that is used to seed
the recommendations should not be included in the returned list.
Fix self-removal from badge recommendations list.
Test recommendations endpoint, add limit.
Update docs
Merge pull request #170 from mozilla/165-recommend-without-user

Add badge recommendations endpoint (for other badges, not users).