Add SimpleCov
List the available Rake tasks to run tests in the README [ci skip]
Reorganize the integration tests a little bit
Add a way to require RR without the autohook
Only autohook is part of the public API here
Use relative requires instead of absolute requires

These are ultimately better to ensure that files do not get required twice (in
exchange for possible lookup ambiguities, which can be resolved using Bundler).
Refactor the Rails integration tests
Add bit about script/run_full_test_suite to README [ci skip]