Change the reference to rails in the 'sync' docs to reflect Rails 4
Merge pull request #3037 from rsofaer/update-docs-rails-4

Change the reference to rails in the 'sync' docs to reflect Rails 4
Update collection docs on underscore methods.
Merge pull request #3040 from braddunbar/underscore-methods

Update collection docs on underscore methods.
Fix #3015 - History#stop cleans up iframe.
Merge pull request #3041 from braddunbar/iframe-cleanup

Fix #3015 - History#stop cleans up iframe.
Return `id` assumption for simple polymorphic `Collection#model` factories.

Also, add tests to make sure it doesn't break in the future.
Make new tests more thorough