Add Slash theme

I added this theme as a submodule in .themes/slash to make it easier to
pull in updates to the theme.

  git submoudle add git://github.com/tommy351/Octopress-Theme-Slash.git .themes/slash
  git submodule update --init
  rake install['slash']
  rake generate

I like this theme because it has a minimal, clean look and is
responsive. More information about Slash:

  http://zespia.tw/Octopress-Theme-Slash/
Install Slash theme

This is the result of running

  rake install['slash']