Added register definitions

Added all possible register definitions from the datasheet. Still have to
work out some of the DIO register configuration definitions. Code will
not compile at this point because of all these massive changes. Also began
adding Doxygen comments.
Added more configuration definitions

These should be the last of the configuration definitions (for now). The rest
of the features will need to be fully implemented in order to know for sure.
Updated code to use configuration definitions

Swapped out old code with the newly added configuration definitions code.
The driver is now compilable again. Tested on Arduino Uno, all features
currently implemented are working again. Began adding Doxygen comments.