Mention tileWidth/tileHeight on custom tile source example

[iangilman]

I think many folks will still want to stick with tileSize, but we should mention that tileWidth/tileHeight is also an option on:

http://openseadragon.github.io/examples/tilesource-custom/

@ConnerMan would you care to take a whack at this?

Note: we won't deploy until the new code is released.