Working clipboard and -f for factory_girl

These are some substantial changes based on my experience with the gem. First, the clipboard now works for Mac and Windows as well as Linux. Note that the Linux option is currently broken upstream, as `scripts` is undefined.

Also, it no longer makes sense to have factory_girl as a top-level output, since most users will want to configure their generators to create factory_girl as part of the regular scaffolding. I explained this in the Readme, gave an example of configuring generators, and moved factory_girl behind a `-f`. This also makes the clipboard more useful, since it's just for a single command.

Thanks for creating the gem, which has been immensely helpful to me, and thanks for considering these changes.