Add optional close button; minor cleanups#31

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driehuis wants to merge 4 commits intotutorialzine:masterfrom

driehuis:master

Commits on Sep 14, 2014

Silence console warnings in Firefox by consistently returning a result from showImage, hideOverlay and showOverlay.
Bert Driehuis
committed
Clean up whitespace (hard spaces introduced by an earlier pull request, it seems).
Bert Driehuis
committed

Commits on Sep 15, 2014

Add an optional close button. Enable by overriding #closeButton{top} in your CSS.
Bert Driehuis
committed
Get the pixel positions of the close button just right.
Bert Driehuis
committed