Added method for multi requests
Added method for multi requests

Added GetMulti() for [multi
requests](https://developer.foursquare.com/docs/multi/multi). It was
possible to do this using only GetPrivate(), but I find this more handy.

Example use:
	$params = array(
		array(
			'endpoint' => 'users/self'
        ),
		array(
			'endpoint' => 'venues/search',
			'near' => 'Chicago, IL'
        ),
		array(
			'endpoint' => 'venues/search',
			'll' => '40,30'
		)
	);
	$results = $foursquare->GetMulti($params, true);

Note: If you're getting 'param_error' errors, like 'Must provide
parameter ll or near', it means your request is to big and you must use
POST.

grigorkh/php-foursquare,gilberg-vrn/php-foursquare,hownowstephen/php-foursquare