Em String dasherize should remove special characters

This implementation is basically the same as [`parameterize`](http://apidock.com/rails/ActiveSupport/Inflector/parameterize) in Rails. 

The last [caching test](https://github.com/emberjs/ember.js/blob/master/packages/ember-runtime/tests/system/string/dasherize.js#L38-L54) is currently failing in this implementation, but I'm not sure how to fix it. 

My use case for this implementation is because I'm using dasherize to build URLs. Not sure if `dasherize` is the appropriate function or not. 

Feedback welcome.