Add Huawei E220 AT Handler

This is based off information from [this blog post](http://myraspberryandme.wordpress.com/2013/09/13/short-message-texting-sms-with-huawei-e220/), at the very end.  This modem apparently has a quirk which makes it return error 330 (i.e. unknnown SMSC) unless `AT+CSCA` is called at least once.  This patch just does what the blog post suggests, which is to feed the value of `AT+CSCA?` back into it.