Add support for Composer, the PHP dependency manager

This will allow the Arena PHP Library to added to `composer.json` as a dependency for a project.

```
{
	"repositories": [
		{
			"type": "vcs",
			"url": "https://github.com/arenahq/arena-php"
		}
	],
	"require": {
		"arenahq/arena-php": "dev-master"
	}
}
```

Additionally the library could be submitted to [Packagist](https://packagist.org/), which would allow the library to be added like:

```
{
	"require": {
		"arenahq/arena-php": "dev-master"
	}
}
```