Correctly handle font face sources

[I3undy]

@Piedone created on Sep 5, 2014:
https://combinator.codeplex.com/workitem/68

Font-face declarations can't be in combined resources if a remote resource storage or different resource base URL domain is used since their srcs should point to URLs under the website's domain.

This means that any CSS with a font-face declaration that has an src pointing to an URL needs either to point back to the site (but this defeats resource sharing) or allow-origin should be configured, see: http://stackoverflow.com/questions/11616306/css-font-face-absolute-url-from-external-domain-fonts-not-loading-in-firefox

To go with the first one and point srcs to the site domain we'd need to detect if we're dealing with the src declaration of a font-face block. This needs ExCSS: #7