Posibility to set multiple devices for one user

[simivar]

PushOver API "Users, Groups, and Devices" states that:

Messages may be addressed to multiple specific devices by joining them with a comma (such as device=iphone,nexus5).

Right now if you use ->setDeviceName('iphone,nexus5') you get an exception Device name must be no more than 25 characters long and contain character set [A-Za-z0-9-]. This PR fixes it. Now you can use:

$user = new User('myID', 'motorola');
$user->setDeviceName('iphone')
     ->setDeviceName('nexus5');

echo $user->getDeviceName(); // prints 'motorola,iphone,nexus5'

A device that is not correct still throws an exception:

$user = new User('myID', 'motorola-over-25-characters');

// Exception: Device name must be no more than 25 characters long and contain character set [A-Za-z0-9-]