Method for finding the correct application model.

This method first checks to see if an application configuration is
defined, then gets the correct ApplicationModel. Reverts to
ActiveRecord::Base if it can't find anything.
Changing comments to use ApplicationModel.

The comments have become outdated since the creation of the
ApplicationModel. Now, the model classes should inherit from
ApplicationModel instead of ActiveRecord::Base.
Renamed ApplicationModel to ApplicationRecord.

This new name is less general and more appropriate since these changes
are limited to Active Record.