Allow fetching user data without posts and fix user add to aspect exception handling

Changes:
* Allow fetching of user data only without posts when creating User, using people.json search mechanism (requires handle).
* Some refactoring, no data saved in User attributes outside data dict.
* Return more precise error description when adding to aspects
* Status code 200 is returned by Diaspora* on a successful addition of user to aspect

I tried to run the tests but I guess they require Python 3.x which I installed (using pythonz) but then failed to get even requests module to fully install under my Python 3.x virtualenv. I can try again tomorrow, and also improve tests at the same time. Submitting this pull for initial review and comments due to some refactoring.