RF24 v1.2.0

• Make internal delays configurable

There are two main delays in the library:

1. txDelay is called during stopListening. This is important when ESB/AA
   is enabled, but can be reduced if AA is disabled or if there is a delay
   between calling stopListening() and any writes to the radio
2. csDelay is called every time the CSN/SS SPI pin is toggled to reduce
   SPI polling. This can be reduced to 0 if interrupts or timed polling is
   taking place.

• Documenation Update