Tessel & RPi compat

[jiahuang]

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 and Tessel.

Tested and working on a Tessel. Did not test on the RPi, though none of the index.js logic changed.

[natevw]

Suwheeet, thanks for doing all this. I'm heads down on the SD/FAT stuff but am excited to test this on RasPi and merge it in!