Create a self-scaffolding XBlock project

This project is meant to ease XBlock creation. It provides a forkable
template that bootstraps itself to create a production-ready XBlock
project with static asset minification/concatenation for HTML, CSS, and
JS.

This project is licensed under the AGPLv3.
Fork XBlock from template
Implement Image Modal as an XBlock

This feature, which previously existed as a Platform HTML template, has
been refactored and revamped into a stand-alone XBlock.

These changes should have a significant effect on client-side
performance, as much of this content had been going across the wire on
every page request. Now, this content will only be loaded on courseware
pages which actually contain an Image Modal.

TODO: The existing image modal code in platform should be removed
_after_ this merges _and_ all previously existing instances have been
converted.
Merge pull request #1

from Stanford-Online:dev
into Stanford-Online:master