refactor: use pratt parser for if expressions - WIP, DON'T MERGE

So, here is the (very) first version of the Pratt's parser being used for expressions contained inside {if} statements. This still needs some polishing and there are 4 skipped tests but opening this PR now for:
* code review
* tests checks in all browsers on Sauce
should add:
* @param {Object} rootscope the parent root scope object containing the reference to the global objects accessible from the template file scope