refactor engine testing scenario

Engine is now tested standalone by leveraging a dummy rails app
Enable Guard for speedy testing
Move factories to the more standard location `spec/factories/*`
Update README with a Testing section
Rename migrations to contain datetimestamps for their version to fix migration order issues when migrating the dummy application

readme.md

@@ -27,4 +27,19 @@ Next, to install the blog plugin run:
 
 Finally migrate your database and you're done.
 
-    rake db:migrate
+    rake db:migrate
+    
+## Developing & Contributing
+
+### Testing
+
+Setup the test environment
+
+    $ bundle install
+    $ bundle exec rake app:db:migrate
+    $ bundle exec rake app:db:test:prepare
+
+Run the test suite with [Guard](https://github.com/guard/guard)
+
+    $ bundle exec guard start
+