make Throttle's dependencies explicit

Following the update to Throttle to support express 3.0, it seems to make sense to declare Throttle's dependencies within the project. npm supports using a 'package.json' file that declares dependencies and users can install them easily using `npm install` from within the root of the repository.

To get Throttle up and running after this would be close to:

     $ git clone https://github.com/dmolsen/Throttle.git
     $ cd Throttle
     $ npm install
     $ node app.js