Subtantially enhanced descriptions in brew.sh

By adding basic descriptions with each tool install command, it's easier
to identify what the program is and where additional information is
available.
Consolidating programs and removing duplicates
Moving tap commands into sections, one each for homebrew and casks
Better description of application aliasing after homebrew run
Uncommenting exiv2 in brew install list

This was commented out in the original brew.sh, and after doing a
git-blame it was unclear why.