Add mouse cursor support

Add a new api, window.set_cursor, for setting the cursor. The enum MouseCursor lists the possible cursors.

Only X11 is implemented. On OSX, Android, & Win32 the window.set_cursor function
either does nothing or calls the "unimplemented!" macro.

src/osx/mod.rs

@@ -491,4 +491,8 @@ impl Window {
     pub fn set_window_resize_callback(&mut self, callback: Option<fn(uint, uint)>) {
         self.resize = callback;
     }
+
+    pub fn set_cursor(&self, cursor: MouseCursor) {
+        unimplemented!()
+    }
 }