Add zoom_factor param to capture() and capture_to().

I needed the ability to capture high-resolution screenshots (analogous to what phantomjs' rasterize.js offers), so I added a zoom_factor parameter to Ghost.capture() and Ghost.capture_to(). I followed the pattern that I saw in Ghost.print_to_pdf(), which also has a zoom_factor parameter. 

I'm sending a pull request in case this helps, but no need to merge in if you don't want this. Or let me know if it should be implemented differently or refactored somehow.