* Always init SPI to 250k to start for SD cards.
* Add ability to configure byte written during read.
* Add ability to read and write to portions of buffers like existing I2C API.
This prevents corrupting previous functional objects by stealing their pins
out from under them. It prevents this by ensuring that pins are in default
state before claiming them. It also verifies pins are released correctly and
reset on soft reset.
Fixes#4, instantiating a second class will fail.
Fixes#29, pins are now reset too.
Docs are here: http://tannewt-micropython.readthedocs.io/en/microcontroller/
It differs from upstream's machine in the following ways:
* Python API is identical across ports due to code structure. (Lives in shared-bindings)
* Focuses on abstracting common functionality (AnalogIn) and not representing structure (ADC).
* Documentation lives with code making it easy to ensure they match.
* Pin is split into references (board.D13 and microcontroller.pin.PA17) and functionality (DigitalInOut).
* All nativeio classes claim underlying hardware resources when inited on construction, support Context Managers (aka with statements) and have deinit methods which release the claimed hardware.
* All constructors take pin references rather than peripheral ids. Its up to the implementation to find hardware or throw and exception.
