Samr21: ADC implementation

New and refactored ADC driver.

As of now we don't have a defined config file for the ADC. This board have got a pretty neat ADC with many features. Most of these features can be switched on/off in the /periph_conf.h file.
e.g. Theres a DEFAULT value defined in the file for most of these non bool featues. fx.
#define ADC_0_GAIN_FACTOR_DEFAULT	ADC_0_GAIN_FACTOR_1X

TODOs:
adc_map
adc_mapf
Why is the API documentation in the implemntation?