Refactor: Adding a place for reference implementations of plug-ins (xblocks, fields, services, etc.)
djpyfs: Adding a prototype field+service for djpyfs, a filesystem-like storage object
pep8/pylint/NedLint/sanitation cleanup
Merge pull request #228 from edx/pmitros/final-pyfs-refactor

Pyfilesystem support+refactor

* Adds pyfilesystem
* Adds a place for reference implementations of plug-ins
* Starts a discussion about how to plug services
Enforce type of the Fields when set

Add a enforce_type method and call it on __set__ to make sure that
the stored value type is consistent.

enforce_type is aliased to from_json for the most basic types
Update tests to test Fields setting in addition to loading
Run enforce_type on the supplied field default
Make enforce_type an optional flag, to avoid conflicts
Add warnings for cases where enforce_type would change behavior
Style fixes for the enforce_type branch