Heartbeat proteins, etc.

Here's what I've been working on. Let me know if you think it all looks good. I tested both examples and they work as expected. Let me know if you have any questions.

#### Overview of changes/additions:
* Added heartbeat protein to Greenduino class (deposits a protein of the state of all its pins every Travail)
* Wrote example using the heartbeat protein (stethoscope.C and operaio.C)
* Standardized protein interface on "to-arduino" and "from-arduino" pools; updated Sample.C accordingly
* Added a basic README for the repo
* Added protein interface to set pin modes and set pin outputs