Clean up 960.css & 960-rtl.css

Nathan Smith brought up some minor issues with the grid styles. Lot's of rules that weren't needed were removed. Basically the "position:relative;" rule applied to push/pull classes were not necessary. Whenever they are used, it should be assigned a .grid-[N] value which already has the positioning rule. This cut down on the number of lines quite a bit.

The ".container-16 .pull-16" ruleset was also removed. It went beyond what was necessary so that's been removed.

There was also a bug within the RTL styles where it was impossible to assign both a .prefix-[N] and .suffix-[N] class to the same element. Those rules have been regrouped so it cascades properly.

This patch below has been committed in both 6 and head.