This PR implements SnowPi, adds the ability for LEDBoard's to own other
LEDBoards as well as LEDs, and enhances blink so that manually
controlling a LED automatically stops it from blinking (no matter
whether it's blinking itself or a LEDBoard is blinking it).
It also fixes up RGBLED and Motor which I managed to break with the last
PR ...
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.
Added source_delay property to SourceMixin which means it'll now appear
everywhere. Default is 0.01 which is just enough to drop CPU usage while
remaining responsive enough for the majority of purposes.
This removes the circular dependency introduced in PR#137. This also
fixes up an issue in the base meta-class which meant it wasn't working
in Python 3 (only Python 2), and adds a bit to the meta-class to allow
docstrings to be inherited (taken from the rest-docs branch).
