Feature/make gpio ports configurable

[white-gecko]

Fix #71 .

To actually avoid using the UART Port I'm now using:

bg = BrachioGraph(inner_arm=8, outer_arm=8, pw_up=1400, pw_down=1700, bounds=[-9, 5, 9, 13], servo_1_pin=4)

also 17, 27, 22 works well:

bg = BrachioGraph(inner_arm=8, outer_arm=8, pw_up=1400, pw_down=1700, bounds=[-9, 5, 9, 13], servo_1_pin=17, servo_2_pin=27, servo_3_pin=22)