model: Removed DBSession variable
model/base: Use Flask-SQLAlchemy declarative base
model/base: Removed DeclarativeBase variable
model/base: Added monkey_patch_model() function
model/base: Removed metadata variable
tests: Fixed __all__ definition

url_for doesn't exist anywhere near this module/package
model/airspace: Use ST_Contains for get_info() method
model: Use db.Column et al

Using the Flask-SQLAlchemy implementation adds a few additional
functionalities to the returned queries and reduced the number of necessary
imports.

sqlalchemy.types could be replaced too, but since geoalchemy.types would
still be needed it was kept for consistency.
Merge pull request #75 from Turbo87/db-session

Replacing old tg database constructs