Shoes

This extracts the packaging code from Shoes4 and reorganizes it as a Furoshiki responsibility. Run specs with `rspec spec`.

The code is set up so that Shoes requires Furoshiki (as a packaging utility). Furoshiki does not require Shoes...except that, in order to package a Shoes app, Furoshiki needs to bundle a Shoes gem. So, currently, it does not require Shoes (in a `require 'shoes'` sense), but it does require that the Shoes gem is installed in the current environment. Not quite a circular dependency, but it feels awkward. Improvements welcome.

I propose that we merge this in and release a 0.0.3, so that we can add `'furoshiki', '> 0.0.3'` to our Shoes 4 dependencies. I have pushed a [compatible Shoes 4 branch](https://github.com/shoes/shoes4/tree/extract-packaging).

Cheers!