Creating a template for ApplicationModel.

The application model will keep all of the Active Record configurations,
instead of sending the configurations directly to ActiveRecord::Base.
This means it will be possible to configure multiple applications if
each application has its own ApplicationModel.

Also, changing some tests to correspond to these changes.
Adding ApplicationModel to the test helper.

The Active Record tests should now include inside of it an
ApplicationModel.
Using ApplicationModel in Active Record tests.

This replaces the current use of ActiveRecord::Base in most tests.
Changing the models.
Creating ApplicationModel methods.

These methods will be used by the ApplicationModel to send the correct
configuration.