Expand Support and Implement Adafruit_Sensor interface

I have forked @markruys work to add more robust support for register compatible gyroscopes like the L3G3200D, L3G4200D, and L3GD20H.  I have also implemented the Adafruit_Sensor interface, bringing this library up to par with the other Adafruit Sensor libraries.  I still need to update the comment headers and readme file to reflect these changes, but I thought I would get your feedback first... this is my first attempt at C++ development and embedded development, so I appreciate any feedback you or @microbuilder can give.