[WIP] Refactored ReplaceStrategies into Commands

Fixes #19, almost completed

Tests are currently failing because I didn't yet updated the 4 `EditorSpec` manipulation methods. I'm not sure how to properly write test with phpspec about correct command being created inside the related Editor's method. Waiting for some feedback on this point :wink: 

Moreover, I'm not really satisfied with the current solution because a `LineContentConverter` is instantiated each time a Command is created.

I hope the current architecture isn't too awful and is now opened for some review :smile: 

- [X] Create commands
- [ ] Fix tests
- [ ] Remove replace engine and strategies




You can use [SplStack](http://www.php.net//manual/en/class.splstack.php), which is more optimized than array for stacks.
However in our case, I'd rather have a simple collection of commands, so an array is fine (you should change the name though, to avoid confusion).