Allow changing passwords

Added capability and tests to db_mongo (the underlying database API).
Added endpoint and tests to support PUT to /user/:userid.

This set of changes:

* Allows updateUser in db_mongo to change the pwhash field by setting a new password (password cannot be empty).
* Add updateUser method to the userapi methods and make it so it checks tokens
* Add PUT /user/:userid to the endpoints and tie it to updateUser