jQuery Forum

I guess that it's a very simple idea. Just make a internal cache that store the `selector` and `context` in an array, and if the DOM not be modified since the last time, use the last result.

I make this `jsperf` and I think that results are veeeeeery good: http://jsperf.com/domsubtreemodified

I know that only some browsers support that, and it's deprecated (will be updated) on API, but it really is too faster then current methods, I guess.