Will/ease integration

Adds two concrete AI algorithm implementations:
* A fake implementation for local testing, which assigns scores deterministically based on the length of the input essay.
* A wrapper for [EASE](https://github.com/edx/ease).

Because EASE requires so many dependencies, I have configured the tests for the EASE wrapper to run only if EASE has been installed in the Python environment (for instance, inside the Vagrant VM).

@stephensanchez @gradyward