Implemented Xblock Runtime User service

@jbau @cpenning

This commit introduces a new Xblock Runtime service: ModuleUserService in
common/lib/xmodule/xmodule/modulestore/django.py

With this new service, it is now possible to store user models as
XBlockUser objects and store desired attributes in them.

Xblocks can now call self.runtime.service(self, 'user') to access the user
service and call get_user() to receive the XBlockUser object of the current
user.

** This PR is to open up a conversation about this feature