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
gem naming is that you'd expect to be able to require 'shoes/core'. So
this commit basically aliases require 'shoes' to require 'shoes/core'.
Merge pull request #972 from shoes/shoes-or-shoes-core

Require as 'shoes' or 'shoes/core'