REFACTOR tests on both rails-3 and rails-4

It is now possible to run tests against both rails-3 and rails-4 :

Rails-4 :

```shell
 # One time setup
bundle install --gemfile Gemfile.rails4
cd test/dummy_rails4
BUNDLE_GEMFILE=../../Gemfile.rails4 bundle exec rake db:migrate
BUNDLE_GEMFILE=../../Gemfile.rails4 bundle exec rake db:test:prepare
cd ../..

 # Then
bundle exec rake test
```

Rails-3 :

```shell
 # One time setup
bundle install --gemfile Gemfile.rails3
cd test/dummy_rails3
BUNDLE_GEMFILE=../../Gemfile.rails3 bundle exec rake db:migrate
BUNDLE_GEMFILE=../../Gemfile.rails3 bundle exec rake db:test:prepare
cd ../..

 # Then
RAILS_VERSION=3 bundle exec rake test
```

Close #8