Nested properties and HAML conflict

I added syntax highlighting for [nested properties](http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#nested_properties) of `font:` (such as `weight`, `style`, `size`, etc.).

I also added a note in the README informing users how to stop Sublime Text from initially interpreting `.sass` files as HAML.

Thanks for working on this!