Add capability to retrieve user information.

[dfederschmidt]

To my understanding there is currently no way of retrieving user information through python-taiga.
A user story has an assigned_to field containing the user id of the assigned person. At the moment I'm manually retrieving the user information through the API but it would be nicer to have this functionality inside this package.

[yakky]

@dfederschmidt User is managed by python-taiga https://github.com/nephila/python-taiga/blob/master/taiga/models/models.py#L107
Yu can just retrieve the user like any other resource.
This is currently not clear from the documentation, but it's going to be fixed by #62
You can currently use this version of the documentation http://python-taiga.readthedocs.io/en/hotfix-generate_docs/#taiga.models.models.User

[menteora]

Hi, thank you for you library.

I've lost a lot of time to understand how work.

Here's sample, if anyone might need it.

api = TaigaAPI(...)
api.users.get(task.assigned_to)