Adding lecture19
Adding mutex example
Merge branch 'master' of ssh://github.com/AllenDowney/SoftwareSystems
Fixing matrix_soln.c
Adding exam2
Merge branch 'master' of ssh://github.com/AllenDowney/SoftwareSystems
Fixing matrix2.c
Merge branch 'master' of ssh://github.com/AllenDowney/SoftwareSystems
Fixed exam2 vector.c and matrix2.c to function as the test desired.