First pass at XBlock for composition

* Implement a basic XBlock for composing. Allows the user to enter text
  and see what they entered.
* Dirty shell script to install the XBlock demos into this working tree
  so that we can use the workbench for early prototyping. This should
  not be used any more once we integrate into LMS, obviously.
Move openassessment_compose to openassessment.xblock, include XBlock workbench at /workbench
Update XBlock dependency to a version that exports workbench and demo_xblocks packages
Add django-extensions to requirements.
Update XBlock dependency to include one that packages workbench and demo_xblocks.
Resolve settings and urls conflicts.
Missing comma.
Remove setup_xblock.sh script (no longer necessary).
Update install instructions so that our XBlocks are registered correctly.
Merge pull request #18 from edx/xblock_experiment

Running XBlock Workbench in edx-tim