Compile to /build/ and remove artifacts

Reintroduces `/build/`, which was already in .gitignore.

`cake build` will produce the unminified developer versions of the compiled batman.js in `/build/`

`cake build:dist` will produce the minified and unminified versions of `batman.js`, `batman.jquery.js` in `/build/dist/`. `batman.testing.js` is also included unminified. The idea is that the distribution tarball we hand out would contain the files in this folder.

@SoapyIllusions @nickjs @kristianpd for review. Feel free to check out this branch and try the build tasks.

note: this breaks the prof tests but the plan is to reimplement them in the coming weeks anyway.