Fix: RPi memory corruption

- Per #46 the read_payload() function was over-reading data into the
user provided buffer
Update readme for installer

- remove sudo from installer instructions, to prevent the cloned
dirs/files from being downloaded as root
- Add D option with example info
Adding SPI enum to RPi python wrapper
Merge pull request #50 from mz-fuzzy/pyRF24fix

Adding SPI enum to RPi python wrapper per #49
Merge remote-tracking branch 'upstream/master'