Send to a specified gateway and list distinct gateway

Issue Summary

Hi,

In some European country and specially in France, customers can keep their phone number when they change of mobile provider... so determine the provider from a prefix is no longer a good solution.

It could be nice if the smsemail_send() function accept a fourth argument to override the gateway... so the gateway could be defined by the user!

Then it will be necessary to choose the good gateway in a drop down box, so we'll need a distinctive list of gateway!

If the gateway value is (I don't know/NULL) then we try to find it with the classic prefix system.