Now, __storm_pre_flush__ is implemented. For now, we traverse the columns and
we set whatever column that is NOT NULL (allow_none=False) and does not have a default
to None. By doing this, storm raises a NoneError immediately.

This solves #28
Updated models documentation.
Merge pull request #31 from nicholasamorim/not_null

NoneError is raised prior to insert in database, resolves #28.
PYM-5 #close Fixed on pull request #31