Added Support for Emergency Priority

Hi!

Pushover has support for Emergency Priority notifications (https://pushover.net/api#priority) so I added support for php-pushover to send out Emergency Priority Notifications (with the 'expire' + 'retry' parameters). I've updated the readme documentation with descriptions on the expire + retry parameters/functions, updated the test.php showing this new functionality and also added comments in Pushover.php explaining what these new parameters do.

After doing some testing, I discovered that even if expire/retry variables are set and the priority is <2, the notification will still send (Pushover seems to disregard the expire + retry variables if the priority is <2). Hence why there aren't 2 seperate curlopt_postfield lines.