Shortened version of unzip that doesn't create sparse arrays

@jdalton informed me that some browsers have problems with spare arrays[1] so I re-wrote `_.unzip` to not create them. In the process I managed to reduce the amount of code and I'm quite pleased with the result :)

[1]: https://github.com/documentcloud/underscore/commit/c3d695313534ad560082380ccd084a199edcccfe#commitcomment-2925557