Cabal build

Based on discussion in #3. Thoughts?

Note:
- `cabal init` wanted to create a license for the project. I picked GPLv3, since that seems to be the de-facto Light Table license, but I don't feel strongly about this.
- Bootstrapping now requires a `cabal sandbox init && cabal install --dependencies-only && cabal build`. This should either be documented or (probably) automated. (`cabal run` might be useful here, since it automatically recompiles when sources change.)