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.
fix author avatar CSS(34x34), project list style
ui.FindUser to ui.Typeahead for general use
enhance author avatar CSS (32x32, 64x64)
fix hive.FileDownloader instance problem
apply author avatar CSS
fix prjmenu
fix page title