WIP: Do not merge yet -- XBlock experiment

This is a variation on #17. @jrbl: I took your branch, renamed things to keep more with XBlock conventions, moved the relevant parts of our XBlock's setup.py to the global edx-tim one, and made changes to the xblock repo to make workbench inclusion more seamless and less hacky. If you uninstall your existing XBlock repo version, install the new hash specified in requirements/base.txt via pip install, and run `python setup.py develop` in edx-tim, you should be able to pull up the workbench for active development at `localhost:8000/workbench` without having to any hacky copying.

I'll put in a PR tomorrow against the XBlock repo to let us filter out the blocks we don't care about in the workbench view, but I figured this should be good enough to get started for now.

Heads up @stephensanchez