spring does not need to be in the Gemfile
add missing require
Generate binstubs for running spring commands

This solves two problems:

1) It's a bit annoying to prefix every command with "spring". Also you
   don't get autocompletion with "spring t<TAB>".

2) The rubygems load-every-gemspec performance problem
   (https://github.com/rubygems/rubygems/pull/435)

See the changes in README.md for more details.