Faster Collection _reset

As discovered in #2098, resetting Collection#models with an empty array is slow and prone to memory leaks (see [SO post](http://stackoverflow.com/q/1232040)).

Setting `this.models.length = 0` shows a manifold (up to about 50x) improvement over master: (jsperf)][http://jsperf.com/bb-faster-array-reset]

Some good comments from @caseywebdev [here](https://github.com/akre54/backbone/compare/array-reset-speed-testing).