Fold this repo into XBlock

[kdmccormick]

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.

[kdmccormick]

FYI @feanil