Small change in PDU regexp

On some modems CMGR look like 0,"",... or 0,"71231231234",...
Add send flash sms
Add send flash sms pdu DCS
Fix send flash sms
Fix concat header for UCS2
Merge remote-tracking branch 'boomer74/master'

Conflicts:
	gsmmodem/modem.py
Add test case for flash SMS