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.

README.md

@@ -58,14 +58,16 @@ fn main() {
  - Some events are not implemented
  - Implementation is still work-in-progress
  - Vsync not implemented
-
+ - Changing the cursor (set_cursor) is not implemented 
 ### Win32
 
  - You must call `glFlush` before `swap_buffers`, or else on Windows 8 nothing will be visible on the window
  - Pixel formats are not implemented
+ - Changing the cursor (set_cursor) is not implemented
 
 ### X11
 
  - Some input events are not implemented
  - Pixel formats not implemented
  - Vsync not implemented
+ - Not all mouse cursors are implemented (ContextMenu, ...)