- Updated all SPI methods that write bits, so they clock the
  data on the correct clock transition.
- Added tests with a small logic net simulation to
  verify bits are read and written correctly.

Move SPI bit writes to the right clock phase.

- Also fixed a corner case with I2C checking whether the device
  acknowledged sent data.
- Added tests to cover most of the major operations.

Fixes #16

Add I2C clock stretching.