Map optimization : each boiler

Was playing with this pattern a couple weeks ago and look forward to hearing thoughts from other contributors as we're sacrificing terseness for speed via with boilerplate. I've applied this pattern to some of the [other methods in this branch](https://github.com/megawac/underscore/compare/funhouse) + some thoughts from @jdalton (see comments at the bottom for the relevant jsperf post changes for other functions ).

Figured I would open the conversation as its one of the most used functions in the lib and optimizations here pay dividends. Relevant jsperf below

http://jsperf.com/pr-1689/9

Also, I'm mixed about this change and can agree if we reject this :)
You shouldn't test `keys` in the loop. For optimum performance, we should pull the `keys` test out and have two loops: one that accesses `keys[index]` and one that uses `index` directly.