In particular the `pi_revision` thing in PiGPIOPin, all the stuff @lurch
picked up in `pins/data.py` (thank goodness *someone's* watching!), and
make all those links pointing to "Notes" point somewhere useful like
"Pin Numbering"...
Overhaul the pi_info system:
Pin factories are now capable of generating pi_info themselves (although
currently they all just look up the revision and call pi_info with a
specific one).
PiGPIOPin will now return pi_info for the remote pi which can be
specified by parameter or implicitly by the environment vars.
Overvolted Pis should work properly no matter what (some argument over
whether the revision 7 or 8 chars in this case; both should work). Added
some minor tweaks for the new camera-capable Pi Zero
Finally, added a bunch of tests for pins.data
Related to @lurch's comments on #148, this PR contains a database of
pins for each Pi revision, along with various other bits of miscellany
(I might've gotten a bit carried away here...).
Any corrections/extensions welcome!
