Building Tags API #14

* Setup tag validation and filters
* Add Tag_Model::for_api()
* Add unique index to tags table
* Add basic Tags controller
* Add tags feature test
* Add tags fixtures
Add tag search function #14

Allow search by type, parent, tag and tag w/wildcard (q)
Tags: Better handling of 'parent' field

* Allow parent id, parent slug, or parent array
Fix stray : in validation error messages

: was missing from replacement :errors token
Merge branch 'master' into api-tags-14
Make Tags only unique within each type and parent group
Posts: check for tags by id and slug, rather than name.
Posts: return tags as id/url combination
Add color, description columns, and make type NOT NULL.
Add api handling and tests for color and description fields
Add -n to automatically generate tag slugs if they already exist
Fix duplicate tag error during test run
Merge pull request #49 from ushahidi/api-tags-14

API /tags/:id