Support for sending to group chats

I've added support for sending messages to group chats.
The JID for group chats consists of the JID of the group owner, followed by a hyphen and an id. The server for group chats is 'g.us' instead of 's.whatsapp.net'. 

My change checks for the existance of a hyphen to determine whether the recipient is a group chat. If so, it uses 'g.us' as the server.

In case this is not known: The easiest way to find the JID for a group chat is to find the profile image in /sdcard/WhatsApp/Profile Pictures. The JID is the name of the image. (Without the extension of course)