StepFish: fischertechnik I2C stepper motor controller

StepFish is a stepper motor controller for the [fischertechnik construction toy system](https://www.fischertechnik.de/en) based on an ATmega328 microprocessor. It can be used as a standalone controller (and I2C master) or as an I2C slave. Hardware features:

* __up to four steppers__ driven by [StepStick](https://reprap.org/wiki/StepStick)/[Pololu](https://www.pololu.com/product/1182) or compatible carrier boards (e.g. A4988), connected via pin headers. Make sure to limit the carrier boards' current (see below).
* fischertechnik compatible connectors for __two active low endstops__ per stepper axis
* __three PWM pins__ for custom uses like servos, LEDs, or interrupt pins in I2C slave mode
* __I2C connector__ compatible with the [ftDuino](http://ftduino.de/) and fischertechnik TX controllers.
* __serial connector__ for programming or monitoring the ATmega328 with [this kind of USB-serial/TTL/FTDI adapter](https://www.ebay.com/sch/i.html?_nkw=arduino+usb+serial+adapter)
* fischert