Add shoes-package (extracted from furoshiki)

This is the companion to shoes/furoshiki#14. It extracts the shoes-specific code from furoshiki, so that we have no more circular dependencies. Furoshiki is generic and doesn't depend on Shoes.

Not completely certain that I've got the abstractions right, or that the use of inheritance vs mixin vs duck typing is ideal yet. Mainly, I'm shooting for separation between shoes and furoshiki. We can improve the interface at a later point. The idea is that Furoshiki has the general logic for packaging jars and apps (so far), and shoes-package injects the shoes-specific requirements/handling into the furoshiki workflow. Other frameworks could inject different requirements/handling

I suspect this needs some love regarding the versioning fixes that @jasonrclark recently pushed.

Comments???
no sure about the structure of the rest of the file, but this seems like we don't include it in our usual `rake spec` run - might be desirable as we don't work on packaging all the time, the specs are reasonably isolated and the specs take rather long to run.

We should at least have travis run them, though.. 

But maybe they are run. I'm bad at rake.