list_devices: prints serial when discovering ppk2

[wlgrd]

This commit adds an extra field to the return from list_devices with the serial number of the discovered device. Added printing of the serial in the examples, as requested in #41.

Output from example.py

> py example.py
Found PPK2 at /dev/ttyACM0 with serial number C10A56B6
Average of 512 samples is: -1.3063051170568183uA
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
(...)

[wlgrd]

Converting to draft as it is not fixed for windows yet