Move executables to shoes-core

shoes is just an installation shell. We want a backend to be able to get
everything it needs--primary shoes, backend picker, etc. without
dependneing on that default installation.
README updates based on changes to shoes and shoes-core usage
No, really, don't commit shoes-backend
Fix for running from source on Windows
Windows seemed cranky about shoes --version when unqualified
Adding pry-nav to dev Gemfile

Often find this useful, so let's default it.