Moves ORM specific code to lib/impressionist/:orm/models. Adds mongo_mapper models.

Adds config/initializers/impression.rb file on install.

test_app/config/initializers/impression.rb

@@ -0,0 +1,5 @@
+# Use this hook to configure impressionist parameters
+Impressionist.setup do |config|
+  # Define ORM. Could be :active_record (default) and :mongo_mapper
+  # config.orm = :active_record
+end