Add missing unique indices

Referenced in #294

Note that because indexed fields are added in order, the attached commit includes the index on :tag_id and the compound index on taggable_id, taggable_type, context

I was running consistency_fail on my project and it pointed out that since the app enforces uniqueness on these fields, the database should as well. (It's more reliable race-condition-wise, too)

This commit is not ruby 1.8 compatible.  Based on the README, that seems appropriate