Added functions to support a lot more registers

The following registers can now be set/got
 - Frequency hopping (step size and channel selection)
 - Frequency deviation
 - Data rate
 - Modulation Type
 - Modulation Data Source
 - Data Clock Configuration
 - Transmission Power
 - GPIO Function

Registers addresses and some values are now stored as enums in rfm22b_enums.h

-Wall added to Makefile

readByte and writeByte helper functions renamed to getRegister and setRegister and made public. Useful for setting registers directly

Warnings when values out of range replaced by saturations (i.e. if (x>5) x=5)

rfm22b_test.cpp updated to demonstrate all new functions

Readme has not been updated yet...