Callback for inbound sms - messages received

Part of extension work CRM-9914, will allow specify our own handler (and path) which would have code like:

$provider = CRM_SMS_Provider( );
$provider->inbound(array(
'from' => '12223334444', // constituent's mobile number
'to' => '12223334444', // server's phone number
'body' => 'I like to send messages to CiviCRM!',
));

and would be responsible for invoking inbound process.

Any inbound sms gets created as a new activity for the contact, the sms is received from, with subject as 'SMS Received'.

We need to decide on how to create contacts for sms which doesn't have anything more than a number and text. Creating a contact require more than just phone number. For now we will log such sms, which we can't find the contact for.