Updated Pow documentation, so that you can get it running.

If you install pow through homebrow, it crashes with a cryptic node exception of fs.js (see https://github.com/37signals/pow/issues/298)

This is because the ~/Library/Application\ Support/Pow/Hosts directory is required for pow to startup (see http://jerryclinesmith.me/blog/2012/08/07/installing-pow-via-homebrew/)

This patch adds these steps to the documentation.