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
hehe what?

It looks right on my Sublime Text... But i think it is because i trimmed the file with Kate just before commit perhaps tab has got a different meaning there...

I fix it