Improve add contact dialog

This pull request contains some commits discussed in pull request #13. It contains only those changes to the "Add contact" dialog were most people could agree on. Three changes were made: 1. Group field has been moved to bottom 2. Tool tips have been added and 3. example prompts have been added to the input fields. This is how the dialog now looks like:

![jitsi_improved_add_contact_dialog](https://f.cloud.github.com/assets/112226/1937942/b87f961e-7f36-11e3-8138-51654c33754b.png)

All other changes from #13 remain there for discussion.