Clean up OS X instructions, remove old skool plist examples

These plists from the wiki were pretty old. I experimented a bit, cleaned the
puppet agent one up, and pasted it inline in the os x install page. The puppet
master one is irrelevant, because you shouldn't be running puppet master with an
init script.
Improve some wording in pre-install
Clean up Windows install instructions

In this commit, I'm kicking out some stuff that's actually reference about
Puppet's run environment on Windows. I plan to reinstate it elsewhere.
Add upgrade instructions to each cleaned page
Minor tweaks on existing pages
Clean up gems install instructions
Clean up tarball install instructions