URL getter & setter for local Usergrid installation.

We are preparing to launch Usergrid on own infrastructure thus we followed those steps: [Deploying to local Tomcat & Cassandra](http://usergrid.readthedocs.org/en/latest/deploy-local.html)

We are planning to use Usergrid on web & mobile. Since on web we use PHP, I had no way to alter `\Apache\Usergrid\Client::$url` value. I created two simple methods:
* `\Apache\Usergrid\Client::getUrl` (just a getter)
* `\Apache\Usergrid\Client::setUrl` (setter using `filter_var()` to validate URL; throwing `UGException()` exception on failure).