docs(lib): cross linking of resources/activities

This makes it so much easier to get to the example call you are
interested in.
It's getting there, slowly ;)

src/rust/cmn.rs

@@ -34,3 +34,11 @@ pub trait NestedType: MarkerTrait {}
 /// A utility to specify reader types which provide seeking capabilities too
 pub trait ReadSeek: Seek + Read {}
 impl<T: Seek + Read> ReadSeek for T {}
+
+
+/// A utility type which can decode a server response that indicates error
+#[derive(RustcDecodable)]
+struct JsonServerError {
+    error: String,
+    error_description: Option<String>
+}