Add a script to upload my public key to GitHub.
Add the sensible-browser and xdg-open commands to OS X.

They're both just aliases to OS X's "open" command.  This way I can paste code
for Linux into the Terminal on my Mac and it'll work.  For example:

    sensible-browser http://www.example.com

The sensible-browser (and it's siblings: sensible-editor and sensible-pager)
command exists on Debian and its derivatives since at least 2010.

The xdg-open command exists on systems running Freedesktop.org software since
2006.  This includes many Linux distributions.
Add the sensible-editor and sensible-pager commands on OS X.

This way I can paste code for Linux into the Terminal on my Mac and it'll work.

The sensible-browser, sensible-editor, and sensible-pager commands have existed
on Debian and its derivatives since at least 2010.
Add a license section to the readme.
Add a script to set up all the things.
install-git-completion: Check if it's already installed.
install-git-remote-hg: Check if it's already installed.
install-php: Check that both PHP and Composer have been installed.
create-key-pair: Check whether a public key already exists.

This isn't strictly necessary.  OpenSSH already checks whether the key exists
before overwriting it, but it goes to the trouble of generating a key first
which takes a second.  This change speeds up running the setup script when a
public key already exists.