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