Creating dot-file directory if it doesn't exist

I'd made the assumption that it already exists. This caused an error on new
systems where I was deploying the dot-files.
Moving homebrew related code to os-specific area

It triggered errors on a Unix box understandably
Providing for a debug mode in installers

A non-zero value for the environmental variable 'DEBUG' now creates the debug mode
in the installers.
Copying plugin files to ~/.vim before Vundle runs

I did't want vim to throw an error and gum up the installer.
Removing hard-coding of home directory in vimrc

This caused problems while installing the scripts on a linux box with a
different home directory name.
Merge branch 'master' of https://github.com/gkb/gh-dot-files