Add composer file

Hi,

It would be very useful if your project had a `composer.json` file, like the one in this pull request. The `composer.json` file allows others to include your project as part of the dependencies in their own project, assembling a structure of libraries and useful projects without having to use Git submodules or similar.

Here's some [further reading on using Composer with WordPress](http://composer.rarst.net/).

Thank you.

Simon