Why:

• $ make install is broken on macOS, even with the recent change discussed in Ressurect the /usr vs /usr/local discussion #79
• Students using macOS cannot easily use the CS50 library unless they manually setup.
• Current format of dynamic libraries do not match macOS convention (.so vs .dylib)

This PR:

• Adds Makefile.macos, which specifically builds and installs the CS50 library on macOS.
• Adds pre-built man pages to docs/man and generate_man.sh to save some headache from depending on asciidoctor
• Would allow libcs50 to be built and installed using Homebrew, like so:

$ brew install libcs50
==> Using the sandbox
==> Downloading https://github.com/americanhanko/libcs50/archive/8.1.3.tar.gz
==> Downloading from https://codeload.github.com/americanhanko/libcs50/tar.gz/8.1.3
######################################################################## 100.0%
==> make -f Makefile.macos build docs
🍺  /usr/local/Cellar/libcs50/8.1.3: 14 files, 33.2KB, built in 2 seconds

Sources:

• Homebrew Formula Cookbook
• How to open a Hombrew Pull Request

Notes:

• FWIW, I'm currently enrolled and verified for CS50 2017 on edX.
• Eventually, I think there could be a single Makefile - would likely just require some OS conditionals (which I experimented with), but this seemed like the best approach for now.