Refactor: Change setting of setting of source paths to Configuration

Previously we were passing our source roots in via a method call, but because we need to abstract the Thor commands into subcommands, we now need a global place where we can store our source paths.

We add a new Configuration class which will store this information.
Refactor: Add an abstraction layer between the generator and the runner

We were having issues with trying to get the 'ember' subcommand to be able to
take an argument of the app name.  Mainly this was due to the fact that in
order for `Thor`'s `default_command` to work properly, it had to be used inside
of a subcommand.

We added a `Runner` class which simply delegates to either the `EmberGenerator`
or the `RailsGenerator`, which, now that it is a subcommand, allows
`default_command` to work.
Feature: Wire the EmberGenerator up to create new projects

Using its new status as a subcommand, we can now run `rough_draft ember foo` in
order to create an Ember project named 'foo'.