Prevent duplicate tags when saving a new post.

Thanks @thedod for pointing this out! Looks like an oversight from
when I converted from flat files to a database.
Add migrations to compile duplicate tags
Merge pull request #14 from thedod/mention-fix

Fix weird case: mention of post without title?!? (happened)
fix fabfile to point to requirements.txt
Merge branch 'master' into deployment
Tag cloud
Merge branch 'tagcloud' into deployment