Add ability to report form validation errors

I've added the ability for the gem to report form validation errors. Users can turn on this feature by simply adding a after_validation callback for :report_validation_errors_to_rollbar in their models. Basically I've just added this method to ActiveRecord::Base and have made sure it reports for the correct model name each time.

I have tested this a fair bit with a dummy app to ensure that the messages are hitting Rollbar but I've also written two specs to ensure that form validation errors are only reported if present. This means that users can add the after_validation callback and not worry about having issues when there are no validation errors.

My text editor picked up a bunch of whitespace errors so it looks like I've made commits all over the place. Don't worry about those and know that the green chunks of extra code are what I've added.

Have also updated the docs.