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 given a comparitor case is the more common use case from my experience with the functions.

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
```

[**Updated jsperf**](http://jsperf.com/simplified-max-min/2)