Cleanup Servo code to leverage SoftPWM as a basis

[xtacocorex]

The current servo code is prone to large jumps when sitting idle. Essentially wrap the SoftPWM library with abstractions to control by just and angle. This would make it suck less.

Servo has a constant frequency of 50Hz, 0 position PW = 1.5ms, Neg Max PW = 1ms, Pos Max PW = 2ms.

Refer to this:
https://bbs.nextthing.co/t/chip-io-softpwm-doesnt-work/14317/3?u=xtacocorex

[xtacocorex]

http://akizukidenshi.com/download/ds/towerpro/SG90.pdf