L.LatLngBounds.extend optimization

Hello

I'm working on a KD-Tree MarkerCluster and the analysis of performances showed that L.LatLngBounds.extend 
was the second most costly operation. So I optimized it, and… the code is less beautiful :(

Anyway, you have this pull-request.

And this is the result, the pull request contains the orange version : (Other is Internet Explorer 11)
![2014-01-15 11_21_07-test latlngbounds jsperf](https://f.cloud.github.com/assets/45740/1919285/4fb36090-7dd1-11e3-838f-75800847e019.png)