Extracted some code to a new set_viewport_size() method so it can more easily be reused.
This is useful, for example, when testing responsive designs by dynamically resizing the viewport.
Also removed the "measuring viewport" message as it can be quite distracting and does not provide much value.
Merge pull request #12 from jphalip/set_viewport_size

Add set_viewport_size() method so its code can more easily be reused