Create app

Create django application providing annotator-storage functionality.

https://openedx.atlassian.net/browse/TNL-780
Nitpick: could we put the test output files (coverage, diff-quality, etc) into a 'build' directory? That would just help with some consistency in some of our repos. See this Makefile, for example: https://github.com/edx/edx-analytics-dashboard/blob/master/Makefile