API: Simplified code usage (#547)

This addresses changes described at gh-547.

I will add documentation to README momentarily.

Also fixes #568.

Other than that, we need to:

- [x] Add tests for custom importers
- [x] Fetch version from libsass interface #548.
- [ ] <del>Make tests pass on TravisCI.</del> (problem persist, read below for details)

//cc @jhnns