use model class' query object

this allows custom query classes with extra filters on the model object to be used
use model class' query object only if it's present

sometimes it isn't present. the tests now pass.
use model class' query object in views as well

now the logic in `views._get_or_create` matches that in `search.QueryBuilder.create_query`
replace code to use model.query or session.query(model) with helpers.session_query function
Merge pull request #133 from glyphobet/master

Use custom query classes on model objects
Replaces two identical exceptions with a single one.
Fixes allow_patch_many example code.
Clears up preprocessing and postprocessing code and tests.
Adds documentation for preprocessing and postprocessing.
Removes extra newline.
Clarifies documentation string.
Corrects incorrect versionadded:
Adds documentation.
Adds skeleton for pre- and post-processors.

Conflicts:
	CHANGES
	flask_restless/helpers.py
	flask_restless/views.py
Small bug fix
PEP8 fix
Refactoring calling pre-, post-processor functions

Conflicts:
	flask_restless/views.py
Tests for preprocessor functions
Fix in catching exceptoins
Records the range of issues that this branch fixes.

Conflicts:
	CHANGES