enable SoftSPI for RF24 when using ethernet cards

These changes enable the RF24 library to choose to use either the hardware SPI of the SoftSPI from DigitalIO libary. It seems that both the W5100 and ECN28J60 ethernet shields are giving a lot of problems when using both the ethernet and the RF24 libs at the same time. By selecting SoftSPI for the RF24 this problem is eliminated.

In RF24_config.h the user can either comment out:
"#define SOFTSPI" in which case the hardware SPI will be used otherwise SoftSPI will be selected.

In the same file the user can select the pins to use for Clock, MOSI and MISO. Default they are now 14-15-16.