model: move code from __init__.py to base.py

Having too much code, in particular too much imports, inside a package's
__init__.py is a recipe for circular imports, and considered bad practice in
Python [1]

Move out everything from kallithea/model/__init__.py to a new file
kallithea/model/base.py and adapt the existing imports.

[1] http://docs.python-guide.org/en/latest/writing/structure/#packages