Document public and private API
Ensure this gem is not used with Rails < 3.2, or Rails >= 4.2

Support for Rails < 3.2 has been explicitly dropped, and compatibility
with Rails 4.2 (when it arrives) cannot be guaranteed.
Loosen dependency on RSpec
Use plain Markdown in README
Add integration test for upcoming Rails 4.2
Broaden dependencies to include all expected Rails 4 versions
Prepare version 1.2.2 for release