Timer-driven stepper implementation

[soundanalogous]

Create a timer based Stepper motor implementation for ConfigurableFirmata. The current Stepper implementation does not use a timer since all were already in use by StandardFirmata (at least for an Arduino Uno and similar boards). With ConfigurableFirmata a timer-based Stepper implementation should be possible as long as the user does not enable another feature that uses the same timer.

[soundanalogous]

This issue was moved to firmata/ConfigurableFirmata#27