Adding a Rakefile for packaging, testing, and releasing

[tenderlove]

I've moved the C files under ext/usb. I added a Rakefile so that you can do:

$ rake compile

Or for installation directly from git:

$ rake install_gem

For pushing to rubygems.org (this will make a gem, deploy the gem, and tag your repository):

$ rake release VERSION=0.3.0

I also changed the name of the gem to just usb. Then people can do gem install usb.

I hope you like it!