removed magic number 15...

Cant check on size < window edge -15 because table might go right up to window edge. Need a different way of checking if table is oversized (like if it is bigger than parent container.
only calcs width when necessary

dont recalc width for every "priority" unless num columns hidden is different from last attempt. Saves computation especially because default priority =99!
Update jquery.magiccolumns.js
Merge pull request #3 from japgardatablocks/master

Works with layouts where table is closer than 15px to screen edge. Also faster.