Simplified _.max and _.min

Bringing this back up - the last time I saw this being discussed is [#578](https://github.com/jashkenas/underscore/issues/578).

Curious on thoughts as the comparitor case is the most common use case in my experience and as this optimization only takes place when the first argument of the array is a positive number.

http://jsperf.com/simplified-max-min

Removing the native apply also normalizes the output for some oddities mentioned in [#728](https://github.com/jashkenas/underscore/issues/728) when given a positive number as the first argument  and make the functions more consistent in general (for absurdly large collections)

E.g.
```
_.max([1, NaN]) //1 instead of NaN
```