Add an installer script.

Script and/or install instructions need testing to support:
 - [x] Ubuntu (Bash)
 - [x] OS X (ZSH / Bash)
 - [x] Fedora (Bash)
 - [x] Other *nix (FreeBSD 10 works, but requirements/dependency checks are probably not useful enough)
 - [x] MinGW (works but has some errors, and no autocomplete)
 - [ ] Cygwin
 - [ ] PowerShell