Indicate which SASS files are partials

* See http://sass-lang.com/guide, “You might name it something like
_partial.scss. The underscore lets Sass know that the file is only a
partial file and that it should not be generated into a CSS file. Sass
partials are used with the @import directive.”
* Bootstrap IE7 SASS file is not a partial
Merge pull request #39 from alphagov/sass-partial-naming

Indicate which SASS files are partials