[DT] Improved date parser

Revamp of PR #566

>While writing the DataTable cell editors, I found that I have nothing to parse date inputs,
>the available Y.Date.parse could not parse everything that Y.Date.format produced. So, I improved on 
>Y.Date.parse. It still has some limitations, since parsing always has to deal with ambiguity and might not always 
>work, but at least there is something.
>
>Includes User Guide, API docs and tests.