Optimize _.pick and _.omit

Migrate the trivial optimizations from rejected pr #1639

~~The `_.invert` change is debatable, its about ~25% faster in ff and ~20% slower in chrome to the current method. I think it looks nicer than the string map :). [Relevant jsperf](http://jsperf.com/invert-vs-map)~~

~~JSPerf of the changeset vs current master: http://jsperf.com/underscore-1639/5~~