Add ability to double click on an element

[gprasant]

Spec:

browser.doubleClick(cssSelector);

Performs a doubleClick on the element selected by the cssSelector.

[jkrems]

Implementation note: For promise chain style, this would be browser.doubleClickOn(cssSelector).

Also, browser.getElement(cssSelector).doubleClick() should work in the sync driver. Currently it's promise chain only via wd's existing method.