JSHint/JSCS happiness
fix crash: use startX/Y instead of pageX/Y in dragstart event!
fixup/update for the new header .l/.r classes: now named .hl/.hr to easily differentiate between these and their grid cell .r/.l equivalents.
also kill the timers in the plugins when they (or their slickgrid instance) are destroyed.
The new header/cell column resize logic which edits the .l/.r and .hl/.hr classes, turns out to be very fast (at least on a modern browser). This leads to the removal of the now obsolete `options.syncColumnCellResize` option as from now on it will be ALWAYS ON.

The plugins which addressed this option have adjusted accordingly.