Fix case of "OverlayUnwatched.png" directory

The "ListItem.Overlay" will return the value "OverlayUnwatched.png" for
an unwatched item, NOT "OverlayUnWatched.png". On a case sensitive file
system the unwatched overlays will not be shown

See:
https://github.com/xbmc/xbmc/blob/master/xbmc/guilib/GUIListItem.cpp#L207
Merge pull request #60 from EvanPurkhiser/fixup/case-sensitive-unwatched-overlay

Fix case of "OverlayUnwatched.png" directory