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!
This PR adds a software SPI implementation. Firstly this removes the
absolute necessity for spidev (#140), which also means when it's not
present things still work (effectively fixes#185), and also enables any
four pins to be used for SPI devices (which don't require the hardware
implementation).
The software implementation is simplistic but still supports clock
polarity and phase, select-high, and variable bits per word. However it
doesn't allow precise speeds to be implemented because it just wibbles
the clock as fast as it can (which being pure Python isn't actually that
fast).
Finally, because this PR involves creating a framework for "shared"
devices (like SPI devices with multiple channels), it made sense to bung
Energenie (#69) in as wells as this is a really simple shared device.
