Fixed a problem with beforetablesort

Hi Joe,

I've fixed a problem with the `beforetablesort` callback function. It wasn't showing the loading message until *after* the table was sorted - not very helpful! So I've changed the plugin to make it sort asynchronously by using `setTimeout`. This also prevents the browser locking up during sorting.

I also added a new example file with a huge table in it (600 rows, made with an online generator). This showed off the `beforetablesort` problem quite well as it takes a second to sort the table.

I also added a doctype to the example files and cleaned up some of the code (some things were indented 2-3 times more than they needed). Sorry the diffs ended up looking a bit crazy :/