Check that $el is defined for #resizeParent()

[bj-mcduck]

This is a fix for #13, but I have no idea how you have your wp-tiles.js => wp-tiles.min.js compression setup.

For some reason, running wp-tiles.js gets Tiles undefined errors. I noticed that at the top of the min file there is a line with var Tiles = {}; and that isn't in the regular wp-tiles.js file.

I've added a small check that $el.offset() exists, and it returns 0 for the tileOffsetTop value if it doesn't. I would've run whatever compression script you do afterwards, but I have no idea how to.

Hope this helps