Functional code for SNES port 2.

Functional code for SNES port 2.

The assembly routine doesn't work.  No idea why; I'll rework it later.
In the meantime, it's a bad idea to poll the SNES in an interrupt
routine, so the interrupt now sets readyToPollSnes every 1/60th of a
second instead, and the actual polling code has been placed in
getButtons().

This will likely all need a rework when I move to support polling two
controllers simultaneously.