Update dialog to add contact

Based on a discussion on the mailing list I did some changes to the dialog used to add new contacts. I re-ordered the controls, renamed the labels, added tool tips. Additionally, I added faint gray prompts to the input fields (which will vanish if text is inserted) to guide the user. Below you will see a screen-shot of the changed dialog.

![jitsi-new-add-account-dialog](https://f.cloud.github.com/assets/112226/1730784/466c5bce-62ef-11e3-9397-0bd9734e3da0.png)