v0.2.1

removed ac_rejection mode functionality, it is slow sampling rate and not interesting for brain-duino use case
updated continuous_conversion_mode, tested and working

implemented are:

setting up input channels (tested)
changing to uni / bipolar mode (tested)
changing data rates (tested)
continuous conversion mode (tested)
reading device ID (tested)
getting current conversion channel (tested)
getting conversion data (tested)
continuous read mode (tested)
single conversion mode (not yet tested)
changing clock mode (not yet tested)