Admin improvements for tags. Resolves #15.

 - Added "created_at" timestamp to "Tags" model
 - Tags are ordered by this field in the admin list view
 - Implemented a tag admin list action that toggles "is_base" field
Merge pull request #36 from dchaplinsky/feature/tag_updates

Admin improvements for tags. Resolves #15.
Merge branch 'master' into development
Fix CR