If a text message arrives from an unknown contact it should be automatically added to the contact list

[atsakiridis]

This should be implemented after #232 lays the basic foundation.

Also, we need to consider what is the best way to populate the 'domain' part of the SIP URI for the newly created contact. Should it be IP from which the message was received or something different? Remember that this SIP URI should be valid for use to make calls and send messages after being added. Also, we don't want it to be re-added if the contact exists already.

[atsakiridis]

Let's move it to BETA5 since it's not critical. BETA4 already has too much content