Add CGDisplayCreateImageForRect function

[mtfcd]

Thanks for your work, this crate is very useful to me.
I have a use case to capture a portion of a display. originally I use the method screenshot, but this method will capture the mouse cursor, after some google I tried to put CGDisplayHideCursor and CGDisplayShowCursor around the screenshot. But when my program is not the foreground the cursor won't hide.
Then I found CGDisplayCreateImage and CGDisplayCreateImageForRect from apple document, these two worked fine.
I also added a image_for_rect method to wrap this unsafe one, just like image which is already there. Because the from_ptr method is from ForeignType trait, and there are some break changes in the foreign-type crates, I think it's a little odd to import a crate that is used by core-display just for this function, and had to keep the same version.
Thanks very much.