Shortens names of test methods.
Moves many helper functions from views.py to helpers.py.
Rewrites get_or_create() code to be more clear.
Style fixes.
Fixes issue #181.

This commit changes preprocessors and postprocessors to have keyword
arguments instead of positional arguments. It also makes them modify
their arguments in-place, so we no longer need the NO_CHANGE sentinel
object.
Changes 'which' to 'that' in documentation.
Fixes typo: boolean -> Boolean.
Add hybrid_properties to list of columns from helper

When performing a post to create a new instance we were not allowing the user to
set values for a hybrid_property since it was not in the set of defined columns.
However, a patch/put request worked just fine to update the attribute. This
change fixes it so you can set the hybrid_property attribute during a post
request.