remove useless variables & support setting duration value as a quotation mark wrapped number (such as '2000')

In `.animate`

jQuery support `duration` set  as a _quotation mark wrapped number_ (such as `'2000'` , string). the behavior as same as it set to `2000` ,number.

But Zepto not support that yet . 

This is the pull request mainly for.

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By the way,

    endEventName, endAnimationName,

are not been used , so remove them.

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And 

    document = window.document,

used only once for the following code :

    testEl = document.createElement('div'),

is not that important, so also can be removed.