Also tell a whole load of lies about what's a keyword-only arg. In the
vast majority of cases, pin_factory (and other args) *aren't*
keyword-only ... but we'd really like them to be, it's just difficult
while maintaining py2 compatibility.
Added notes on how the abstracts are represented, ensured all the class
hierarchies were up to date, and changed the orientation so the classes
are actually readable in the big chart.
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.
