add composer.json for composer support

This would allow addition of the OnApp API via this in a composer file:

``` javascript
{
    "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](https://raw.github.com/OnApp/OnApp-PHP-Wrapper/master/REQUIREMENTS)