HUGE update. Among others, there have been refactoring and
implementation change, a HUGE work on tests and bugfixes. All tests now
pass. 5 files changes all related to SPI. spi_2.*, test_spi.* and
test_runner.c
Restructured spi_2.h
Commented on functions in spi_2.h
Updated some tests in test_spi.c
Added an extra test to test for variable bitlenghts for the transmission
of SPI. Added to the codebase.
Added two more test functions and added to testbase in test_runner.c for
SPI.