* Add GPIO support for the stm32f091
* Implement RCC, Serial and Timers for stm32f091
* Implement ADC, I2C and SPI for stm32f091
* Add STM32F091 to Travis config, CHANGELOG and README
Base implementation of ADC functionality according to the embedded hal
traits for `Channel` and `OneShot`. Also provides references for
converting the internal temperature sensor and voltage reference values,
also the VBat reference where available.
Closes#11.