Added API for resetting user password.
Added traceback to error message.
Added command to get tags for a given model.
Fixed bugs in tags.

Fixed
=====

* Ensured that when removing a tag, the tag is also removed from all models
  using that tag.
* Ensured that tag can only be added to a model once.
* Ensured that only an existing tag can be added to a model.
Merge branch 'master' of https://github.com/csdms/wmt