project: Add Tagging feature.

While getting tags:
* Return 406 if the client cannot accept appcation/json.

While tagging:
* Return the tag if that is tagged newly.
* Create new tag if no tag matches the given name.
* Do nothing if that has been tagged already.

While untagging:
* Respond 404 if no tag matches the given id.
* Delete the tag if there is no project tagged by that.
Merge pull request #9 from npcode/feature/project-tag

project: Add Tagging feature.