adds class_option for passing ember_path from the command line

Since I need to work on an ember app in a big rails project (that already has its assets) I'd like to have a separate directory for the ember app inside the assets directory.

I didn't want to alter the `ember_path` method so I added a class option.

At first I tried overriding `ember_path` in my lib directory but I found that every generator (except from the bootstrap one) does not respect the value specified by `ember_path`.
I ended up adding the missing tests for the generators and the class_option.

I hope it's not rubbish, I'm pretty sleepy right now :smile: