OSX/Linux shell differences in teaching notes

I added this section and included two differences found at the UC Davis workshop and another I've come across. There are a number of others (such as differences in `sed` options), but I've excluded them because they seem too advanced or esoteric to likely come up in these lessons.  Unfortunately I have not found any source that documents these differences comprehensively.
Typo fix
Typo fix: linux -> Linux
OSX section to POSIX, moved to top
Moved, reworded POSIX-compliance note
Merge pull request #594 from noamross/master

OSX/Linux shell differences in teaching notes