ATTiny support fr jscrane fork tested and working

ATTiny support pulled in from https://github.com/jscrane/RF24
- SPI support for ATTiny is included with the library. Do NOT include
SPI.h with tiny.

Pin info etc can be found at:
http://programmablehardware.blogspot.ca/2013/09/rf24-with-attiny84.html

ATTiny 85 Example:
RF24 radio(4,3);    // CE: 4  CS: 3