Updated zepto.js for height correction

Changed the height and width functions to reflect the correct height and width of the document.
Updated zepto.html

Included tests to make sure the height and width functions are using the correct property for the document height and width.
Updated zepto.html

Reverted the width test as the change was inadequate for all scenarios.
Merge pull request #753 from charsleysa/height-fix

Height fix