Add ability to pass in Express app vs. relying on clabot to create one.

This fixes compatibility issues where some modules, such as passport,
require middleware to be added first. Since clabot creates the express
app and adds some middleware immediately, it was previously incompatible
with passport.
Merge pull request #15 from erinspice/optional-express-app-parameter

Add ability to pass in Express app vs. relying on clabot to create one.