Simplify build with rake-compiler.

This requires a quick rename of the directory and .cc file, but means we can let others worry about
defining nice compile and clean targets that are cross-platform and use tmp/. We also need to ignore
.bundle files (which OSX emits rather than .so).

This follows the adivce [from the RubyGems guide](http://guides.rubygems.org/gems-with-extensions/).