device name convention & read-only emulator

[lpxz]

HI,
thanks for making this great work public!

Here are a few fixes:
(1) in boot_devices() in emulator.py, you used the following to get the device name.
However, this naming convention may not always be correct.
for i in range(0, settings.DEVICE_NUM):
device_name = settings.AVD_SERIES + str(i)
One simple fix is to let users specify the device_name in settings.py

(2) in boot_devices() in emulator.py, we need the option "-writable-system", otherwise, the mounting of "/system" may fail. Also, please remove "-no-audio" since it is no longer a valid option.
sub.Popen('emulator -avd ' + device_name + " -wipe-data -writable-system",
stdout=sub.PIPE, stderr=sub.PIPE, shell=True)