py: Add "io" module.

So far just includes "open" function, which should be supplied by a port.

TODO: Make the module #ifdef'ed.
unix file: Implement context manager protocol (for "with" statement).
run-tests: Allow to override python3 command to use via environment var.
README: ARM Linux is supported.

Retested again on Ubuntu 12.04 armhf running in chroot on Samsung
ARM Chromebook (Cortex-A15).
stmhal: Reimplement file support using stream protocol API.

This adds readline(), readall(), etc. methods. Also other fixes to make
Python API compliant and "with" support.