Adding lecture19
Adding mutex example
Remove histogram
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Fixing matrix_soln.c
Adding exam2
Merge branch 'master' of https://github.com/AllenDowney/SoftwareSystems
Changes from class 19
Exam2: Add before/after postscript files
Exam2: Fix vector.c code