Add support for composer as per issue #7

There are a few things that you probably want to consider before merging this:

 - Composer requires vendor namespaces, which moves your dependency up to PHP 5.3.  Doubt if it's a problem but you should know it is there.

 - The class filename and namespaced class name both change for PSR-0 compliance required with composer.

Once this is merged, the library should be submitted to packagist so that people can just install it from there with a one-liner.  You issue releases to them by tagging (I think, I haven't done it but I will help).