Add flow typechecking

[fmoo]

RFC - If you're wondering what #32 might look like, this diff adds support for the "flow" typechecker to professor redwood, and addresses all existing type errors.

Notable new package.json scripts are:

• npm run flow - runs the flow typechecker. Should return 0 and print "No errors!"
• npm run bot - runs bot.js (under babel-node, which is required to preprocess strip typehints)