added support for user-defined path and mountable engines

In case you write a [mountable enigine](http://guides.rubyonrails.org/engines.html), there is a so called dummy directory(test/dummy/) which contains a dummy app to mount the engine during testing and development. With the introduced changes to erb2haml, it takes this additional path into account. I have added this path as a hardcoded default, because it's the default dir which rails creates when generating a new mountable engine. In case of rspec, the path to the dummy app will be `spec/dummy`, but thats something the user has to explecitly define. Thats why i added the posibility to pass a user-defined search path to the rake task(as described in the README). I also refactored the rake file because there was a lot of redundancy. I hope that's ok ;)