Some benchmarks and additional options

Adding some benchmarks to test using `_.keys` vs `for(var key in object)`.
During test, tried doing `_.extends({}, {order: 'asc'}, options)` instead of `_.extends({order:'asc'})`. Including the `{}` slowed down the code by 50%. Closing #4 due to this.

Adding `property` option to sort on property values or properties in the child objects per issue #2.