Tessel & RPi compat

This PR merges index.js to work with both Tessel and an RPi. All Tessel workarounds are removed.

Here are the new usage examples for [RPi](https://github.com/tessel/node-nrf/blob/jh-tessel/test.js) and [Tessel](https://github.com/tessel/node-nrf/blob/jh-tessel/examples/RF24-pingpair.js).

Tested and working on a Tessel. Did not test on the RPi, though none of the index.js logic changed.