make Throttle's dependencies explicit

[adamvduke]

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

[dmolsen]

Thanks for this. Forgot.