Enable third pary providers by specifying their class paths on the confi...

This change allows enabling third party providers by specifying the path to the php class on the configuration file.
Example:
<pre>
"providers" => array(
    "Googleplus" => array(
        "provider_path" => /different/path/to/provider/Googleplus,
        "enabled" => true,
        "keys" => array("id" => "xxx", "secret" => "yyy"),
   ), 
   /** and many more**/
)
</pre>
The .php extension is added inside Provider_Adapter.php