Added tags_on for collections.

Added tags_on for collections. This is far more performant than the tag_counts_on method if you don't require the counts and just need tag names.

At work we noticed that tag_counts_on joins against the taggable objects table, which in our case is quite large. Without this join it speed things up substantially. Figured others might want this functionality.

I added tests for the two methods I added. Let me know if you need anything else. Thanks again for writing a great tagging library.