Made MirfSpiDriver methods pure virtual. Fixes linking errors whe not using size optimization.
Added support for const buffers where applicable.
Minor whitespace cleanup.
Fixed a bug where powerUpTx after powerDown would hang the library. Some very minor code improvements.

Conflicts:
	Mirf/Mirf.cpp
Move documentation from Mirf.cpp to Mirf.h.

Additionally fixed some whitespace issues.