Easily add new number to contacts

When an unknown number is provided in the recipient field, offer an easy
way to create a new contact from this dialog - show a button nearby.

Fixes: https://code.google.com/p/esmska/issues/detail?id=255
Contributed by: tomas kozak <kozakto2@fel.cvut.cz>
Pull request: https://github.com/kparal/esmska/pull/12