Fold this repo into XBlock

This repository is useful to XBlock developers, and it is tightly coupled to XBlock. So, let's merge it into the XBlock repository.

Considerations:

• It should be isolated to a sub-directory and have its own requirements file. The requirements file should be available via extras-require. This way, edx-platform can run pip install xblock[django] (so that it doesn't pull in sdk dev tool packages) and developers can run pip install xblock[sdk].
  • We don't have a paved path for this setup, so this will take some creativity. repo-tools does a more complicated version of this: it has EXTRAS_REQUIRE defined for every subdirectory with requirements. We don't need to go that far, but the example might be helpful.

Start by comparing this repository with the XBlock repository and inspect repo-tools/setup.py, especially its EXTRAS_REQUIRE handling. Define how this repository can live in an isolated subdirectory with its own requirements file and be exposed through xblock[sdk] without pulling SDK development tools into xblock[django]. Done means the repositories are merged and both extras install the intended dependencies.