For loop cache

This pull request adds a for loop cache on most of the loops of the distribution:

The following:

    for (var i = 0; i < whatever.length; i++){
        // code here
    }

is changed to:

    for (var i = 0, _len_i = whatever.length; i < _len_i; i++){
        // code here
    }

It should improve performance on most of the browsers (in most modern browsers the effect is negligible).

I ran all the tests and the examples in the gallery and all seem ok.

Please, could you check this to look for bugs and/or test the performance on several browsers?

I'm checking the performance in the following way:

* To go to the commit before all the changes

    git checkout 25962cc573f8736b332709227d7269d026de3576 .

* to go to the last commit

    git checkout 1acda096a5fa8b7226a0a76873aac4b6edae2775 .

* Run the complete test suite several times (http://127.0.0.1:8000/site/tests/index.html) and check the times:

    python -m http.server

From my test on a linux box I get:

FF32 before changes 28.4s (run 9 times)
FF32 after changes 28.6s (run 9 times)
Chrome38 before changes 32.0s (run 3 times), results are very unstable and the browser crashes after some runs of the complete test suite
Chrome38 after changes 31.4.0s (run 6 times), results are very unstable and the browser crashes after some runs of the complete test suite

Tomorrow I will try to check results on win and IE/FF/Chrome.

Kind regards.