Modify write,startWrite, remove rx buffer flushes

Modified write() to use startFastWrite, then toggle CE after completion
to remove need for a delay on some boards.
Added delay to startWrite for non-Arduino boards.
Removed receive buffer flushes. This prevents an issue in the
RF24Network library, where payloads would be received, but flushed when
a transmission takes place prior to reading.
Should address issues #7 and #11