Composer support (PHP dependency manager)

This adds support for [composer](http://getcomposer.org/) to engineyard-serverside.

Detects presence of composer.json or composer.lock and uses them to install dependencies.

It will emit a warning when a composer.json or composer.lock file is detected and composer is not available.

Composer-specific tests can be run with `bundle exec rake php` - right now this runs one set of tests if composer is available on the test system, and another set if it's not. This is less than ideal, but seems like a reasonable tradeoff against a larger refactor.