Allow Slimmer config array

this Pull request allows a slimmer $config array, by building the api_url, raw_url and zip_url URLs in the constructor since the are usually the same. 

So instead of this:

```php
 $config = array(
        'slug' => plugin_basename(__FILE__), // this is the slug of your plugin
        'proper_folder_name' => 'plugin-name', // this is the name of the folder your plugin lives in
        'api_url' => 'https://api.github.com/repos/username/repository-name', // the github API url of your github repo
        'raw_url' => 'https://raw.github.com/username/repository-name/master', // the github raw url of your github repo
        'github_url' => 'https://github.com/username/repository-name', // the github url of your github repo
        'zip_url' => 'https://github.com/username/repository-name/zipball/master', // the zip url of the github repo
        'sslverify' => true // wether WP should check the validity of the SSL cert when getting an update, see https://github.com/jkudish/WordPress-GitHub-Plugin-Updater/issues/2 and https://github.com/jkudish/WordPress-GitHub-Plugin-Updater/issues/4 for details
        'requires' => '3.0', // which version of WordPress does your plugin require?
        'tested' => '3.3', // which version of WordPress is your plugin tested up to?
        'readme' => 'README.md', // which file to use as the readme for the version number
        'access_token' => '', // Access private repositories by authorizing under Appearance > Github Updates when this example plugin is installed
    );
```

```php
$config = array(
        'slug' => plugin_basename(__FILE__), // this is the slug of your plugin
        'proper_folder_name' => 'plugin-name', // this is the name of the folder your plugin lives in
        'github_url' => 'https://github.com/username/repository-name', // the github url of your github repo
        'requires' => '3.0', // which version of WordPress does your plugin require?
        'tested' => '3.3', // which version of WordPress is your plugin tested up to?
        'readme' => 'README.md', // which file to use as the readme for the version number
      );
```

Most of this was already possible but just not for these parameters, so i turned them into optional parameters to keep backwards capability and allow customization. 
Also   i added a new parameter `branch` which defaults to master but it's nice to have the ability to change that so the zip file downloaded is of another branch.