Add tests

[begriffs]

Add some basic sanity checks with https://github.com/heroku/hatchet

[begriffs]

Test that a "hello world" Yesod, Snap, Happstack app can each be deployed and runs.

Test each in the following contexts:

• Downloading prebuilt libraries/binaries from S3
• S3 not available, build libraries/binaries from source
• Redeploy, using cached assets from last time