Adding composer install ability

This makes it so the install is very easy if using Composer.

composer.json
```javascript
{
  "repositories": [
    {
      "type": "vcs",
      "url": "https://github.com/infusionsoft/PHP-iSDK.git"
    }
  ],
  "require": {
    "infusionsoft/PHP-iSDK": "dev-master"
  }
}
````