zsh: set the fpath earlier so that completions are set up correctly

compinit runs as part of shell-options.zsh, and that's when completions
are setup.  It works by examining the fpath and if it's not fully setup,
then completions may be missed.
zsh: add /usr/local/share/zsh/site-functions to the fpath
bash: update the git-completion.bash script

This is the version that ships with v2.0.4 of Git.
osx: turn off App Nap for iTerm2
osx: add more brew packages to install
Use the git completion provided by Homebrew, if available.