Allow installation with Composer

PHP libraries have a de-facto standard package manager: [Composer](https://getcomposer.org/).

Adding a `composer.json` file and registering the imgix-php library with [Packagist](https://packagist.org/), the central Composer repository, allows PHP projects to automatically install the library and keep it up-to-date as new versions are released.

Moreover, Composer provides its own class autoloading mechanism for all managed libraries. I added notes about this in the README.

Once the composer.json is included in your repository, you will need to submit the package to Packagist:
https://packagist.org/packages/submit

Using the repository URL:
https://github.com/imgix/imgix-php.git

You will need to create an account on Packagist first.

Thank you for providing a PHP library, Composer support will be the icing on the cake!