Provide a path for default xml to load upon creation.
Revert "Provide a path for default xml to load upon creation."

This reverts commit 76bf8031adfb1a9c9619d1cdb49bfa11e18f31e5.
Take advantage of new callable defaults.
Polished up callable support so it works properly.
Merge pull request #86 from Kelketek/master

Provide a path for default xml to load upon creation.
Extract PublishEventMixin to xblock-utils
Extract SeleniumBaseTest to xblock-utils
Use xblockutils in utils.py
Have all resource loading go trough utils.loader instance
Fix unittests after rebase