add composer.json for composer support

[ghost]

This would allow addition of the OnApp API via this in a composer file:

{
    "repositories": [
        {   
            "type": "vcs",
            "url": "https://github.com/OnApp/OnApp-PHP-Wrapper-External"
        }
    ],
    "require": {
        "onapp/onapp-php-wrapper-external": "3.1.0.x-dev"
    }
}

Once added, it could be quickly installed by using composer install, with autoloading set up correctly. This can be tested via my fork by changing the VCS URL to https://github.com/james-lsn/OnApp-PHP-Wrapper-External

This also forces the dependencies mentioned here

No other changes must be made as far as I can see, though you could submit it to Packagist so the custom repository would not be needed.