New rule proposal - Detect non-bulletproof font loading

[Jonathan Klein]

What do you think about adding a rule to check for the proper use of @font-face to prevent 404's in Internet Explorer, i.e. http://www.fontspring.com/blog/the-new-bulletproof-font-face-syntax?  

If this is something that you would accept I'll start working on it.  

-Jonathan

[Jonathan Klein]

I went ahead and wrote this since we need it internally - pull request submitted:

https://github.com/stubbornella/csslint/pull/331