Added render callback argument

[shamasis]

This PR modifies the $.fn.render function to add a third callback argument. When a function is passed as the third argument, the function is executed in the rendered element's context upon successful application of the template. This is useful in chaining template loads.

The $.when( /* ... */ ).done() can also be used to arrive to the desired effect, but this is more intuitive for beginners and for rapid prototyping with lesser codes.

Lint checked and README updated.

[shamasis]

Ah. Its late, but I did some changes to use $.when if callback was available. @frkr