Require as 'shoes' or 'shoes/core'

For backwards compatibility and simplicity, we should allow people to
`require 'shoes'`. However, I think what's confusing about the shoes-core
naming (see #951)  is that you'd expect to be able to `require 'shoes/core'`. So
this commit basically aliases `require 'shoes'` to `require 'shoes/core'`.

Thoughts?