Major rework.

Read card identifier to know its parameters.
Implement card unlocking protocol.
Use the same frequencies as in libgc2.
Implement event-based wait for card operations using INT line.
Pythonify the source:
- declare a class
- use enum and ctypes for flag constants and structure definitions
Unhardcode SPI bus and GPIO for INT signal, to be board-independent.
Switch dependencies to pure-python and board-independent modules.

Author: vpelletier