Reverting source.models to 572ad66c

This restores the post.content relationship. Post is no longer a subclass of
Content, and Email is no longer a subclass of Post.
Merge branch 'source-models' into develop

Conflicts:
	assembl/source/models.py
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Splitting source.models out into seperate modules.

Before source/models.py gets too big.
Using explicit import paths in source/models/.

Instead of using relative import paths, I'm referencing the assembl package,
which is available when running the project from the outside with pshell,
pserve or other p* commands.
small bugs
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
username in views
Merge branch 'develop' into tests_refactor
Fixed tempalte for testing.ini