support setting duration value as a quotation mark wrapped number (such as '2000')

jQuery support `duration` set  as a _quotation mark wrapped number_ (such as _string_ `'2000'` ). the behavior as same as it set to _number_ `2000` .

But Zepto not support that yet. 

It has been well tested by myself.

_This is a independent pull request for duration enhancement in #1059_