Keep cursor centered in window while captured.

Previously, cmd-tabbing out would result in the cursor position maybe
ending up outside the window, so if one cmd-tabbed back in with the
cursor still outside, any click would go outside the window and cause
it to lose focus. Not really a good thing. So, this is a bit of a hack,
but it works.
Add GLFW_CURSOR_HIDDEN.

Uses addCursorRect:cursor: as discussed.. well, too long ago. Anyhow,
this will effectively hide the cursor while it is inside a window with
GLFW_CURSOR_HIDDEN or GLFW_CURSOR_CAPTURED enabled. This shouldn't mess
up cursor retain counts either, unlike previous uses of the hide/unhide
methods on NSCursor.

It does allocate a small, single-pixel image for an invisible cursor,
as well as the cursor itself, but that shouldn't be too much trouble.
Make resetMouseCursor consistent with setCursorMode
Merge pull request #52 from nilium/osx-cursor-hidden-patch

Added support for hidden cursor mode on OS X.