Add functions to query IP and MAC

[ghost]

This commit adds two functions to query

• the IP of the module in client mode
• the MAC of the module in client mode

Both functions rely on "AT+CIFSR".

[ekstrand]

Nicely done! What version of ESP8266 firmware have you tested it on?

[ghost]

hey, tests during development were done on:

AT+GMR AT version:0.21.0.0 SDK version:0.9.5

OK

Should be the latest available version of the firmware.