Auto merge of #119 - jrmuizel:cfurl-fs, r=jdm

Add CFURLGetFileSystemRepresentation

<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/core-foundation-rs/119)
<!-- Reviewable:end -->

core-foundation-sys/src/url.rs

@@ -124,7 +124,8 @@ extern {
     //fn CFURLCreateStringByAddingPercentEscapes
     //fn CFURLCreateStringByReplacingPercentEscapes
     //fn CFURLCreateStringByReplacingPercentEscapesUsingEncoding
-    //fn CFURLGetFileSystemRepresentation
+    pub fn CFURLGetFileSystemRepresentation(anURL: CFURLRef, resolveAgainstBase: Boolean, buffer: *mut u8, maxBufLen: CFIndex) -> Boolean;
+
     //fn CFURLGetFSRef
     pub fn CFURLGetString(anURL: CFURLRef) -> CFStringRef;