Get window size

[SebastienBoisard]

It can be useful to be able to get the browser window size.

So, I added 2 functions GetSize() in window.go and page.go (but I'm not sure to comply with method naming convention with GetSize() in page.go).

I tried to add some tests, I hope it's ok, but I'm not used to Ginkgo yet :wink:.

[sclevine]

I merged the Element version of this before I noticed, but it appears that the /size and /location endpoints have been replaced with /rect endpoints for both element and window. I'm leaning towards merging this anyways, given that we already have SetSize, but we should probably introduce GetRect and SetRect for both element and window as well.