Make captureSelector aware of the page zoom factor.

Fixes the captureSelector method not being aware of the WebPage.zoomFactor feature introduced in PhantomJS 1.6 and supported since #157. Previously, captureSelector failed to scale a capture region’s coordinates to match the zoom factor, resulting in incorrect screenshots.

Fixes #917.

## Changes
- Check for casper.page.zoomFactor when getting element bounds, and scale the clipRect accordingly
- Add tests

## Notes
- Also fixes a statement that assigned `Casper.prototype.getElementsBounds = function getElementBounds(selector)`, the right side name not matching the plural `Elements`.