fixes sass --watch for mac osx maverick and other bits

Ran into several issues in customizing the theme using sass --watch  this patch contains the following adjustments.

* Updated instructions in Readme.md for setting up sass and fixing [mac osx maverick](http://stackoverflow.com/questions/22413834/getting-error-when-using-command-line-for-sass-to-watch-files)
  * Instructions include how to install bourbon in the correct path
* [Bourbon Deprecation box-shadow](https://github.com/thoughtbot/bourbon/issues/156) warnings issued during `sass --watch`
* `default.hbs` links to `screen.css` which doesn't reflect the correct changes from `sass --watch` instead use `uno.css`.  Curious if I'm missing something with `screen.css`?
* Add `.gitignore` so that we don't include `bourbon/bourbon` installation as this should always be handled by developer to ensure latest bourbon.