[DT] Improved date parser

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.