bugfix SPI class, change API

bugfix SPI class, change API

For the weird SPI modes 1,3, SK must be in the opposite state than idle before
clocking in/out data.  The constructor uses a different data structure for
argument 'cs'.  A tuple of (mode, pinstate) is provided for each key, e.g.:
    {'_mask': (-1, 0xff),
     '_idle': (-1, 0xff),
     'dev1': (0, 0xf0)}