Refactor _.some using _.every

All tests pass. I only refactored the _.some function.

Before:
* Iterate over collection, return true as soon as any predicate function returns true.
* Code redundancy in _.every and _.some (ll. 208 - 214)

```javascript
var length = obj.length;
var index, currentKey, keys;
if (length !== +length) {
  keys = _.keys(obj);
  length = keys.length;
}
```

After:
* Check if *reversed* truth test fails on every item inside collection.

Advantages: Cleaner, shorter, reuse of _.every, no additional corner case checks inside _.some
jsPerf: http://jsperf.com/refactor-some-using-every

This should not be merged. It's much slower (about 50 %). Sorry for not benchmarking it.