The a-gsm via USB communication it's valid under RASPBIAN OS. Tested on RPI2, 2015-02-16-raspbian-wheezy, USB communication working great!
After USB cabling you will notice in the /var/syslog:
... raspberrypi kernel: [ 2481.862240] usb 1-1.3: cp210x converter now attached to ttyUSB0

Under RPI, must be the same behavior!

In Python files, you must set:
agsm = serial.Serial("/dev/ttyUSB0", serialSpeed, timeout=1)
instead of,
agsm = serial.Serial("/dev/ttyAMA0", serialSpeed, timeout=1)

In this way, you can use in very convenient way, RPI + a-gsm + any other DEVICE/SHIELD with serial communication!!! 

Anyway, the a-gsm powering via RPI USB it is not working (the current supplied by the RPI USB controller it is insufficient, seems to be a RPI design limitation)... you can still power the a-gsm via GND and 5V or Vin pins.

We suggest to you to use the newest RASPBIAN versions. https://www.raspberrypi.org/downloads/

As backup, if something goes wrong, you can still use the 2015-02-16 version. You can download it from here:
http://itbrainpower.net/a-gsm/downloadables/2015-02-16-raspbian-wheezy.zip

Enjoy,


May 25'th, 2015