Add Django Model EndpointSet mixin

[grahamu]

What does this PR do?

Creates mixin with commonly used code for EndpointSet.prepare() to retrieve a Django Model-based Resource object, if the request method is appropriate.

Where should the reviewer start?

Read the mixin code.

How should this be manually tested?

No tests yet for the mixin.

Any background context you want to provide?

See #10

What ticket or issue # does this fix?

N/A

Definition of Done (check if considered and/or addressed):

• Is there appropriate test coverage?

Yes, updated test case ResourceEndpointSets to use new DjangoModelEndpointSetMixin.

• Does this PR require a regression test? All fixes require a regression test.
• Did you update internal documentation appropriately?
• Did you update (or alert the appropriate person to update) user-facing documentation?

[coveralls]

Coverage increased (+0.5%) to 61.847% when pulling dbd2311 on grahamu:django-prepare-mixin into dbf9802 on pinax:master.

[coveralls]

Coverage increased (+0.6%) to 61.898% when pulling 31c04e7 on grahamu:django-prepare-mixin into dbf9802 on pinax:master.

[coveralls]

Coverage increased (+0.6%) to 61.905% when pulling 711a03c on grahamu:django-prepare-mixin into 092e800 on pinax:master.