stmhal: Add simple README.txt to freshly-created filesystem.
stmhal: Add I2C.scan method, to scan all devices on the bus.

Simple way to find the address of an attached I2C device.
stmhal: Add ADC function to read data at a given frequency.

Reads ADC values into a bytearray (or similar) at a fixed rate.  Needs a
better name and improved API.  Also fix up DAC dma function (which also
needs a better name and API).
stmhal: Fix USB MSC so that it unmounts correctly on Mac OS X.

Mac OS X sends a SCSI command to remove the medium when it unmounts a
drive.  If this command is not honoured, then OS X will automatically
remount the drive, making it impossible to eject.  This patch disables
the USB MSC when the right SCSI command is sent.
Merge branch 'master' of github.com:micropython/micropython