Feature Request: Support setSize() to enable CSS breakpoint testing.

[dura-twc]

Currently, on Mac OS X, Opera is the only browser that doesn't support driver.manage().window().setSize();

In fact, on Mac, Opera opens in "fullscreen" mode, which further exacerbates the issue since there's no way to exit fullscreen mode as ESCAPE doesn't work: #85

In this age of "Responsive Web Design", testing CSS breakpoints is imperative, and extremely useful when combined with screenshots for a PDiff test!