Add rasterisation options

Adds several options to the screenshot service:
 - Delay screenshot until a JavaScript expression evaluates truthy in the target page. For instance, the expression could check if a certain class has been applied to an element. This is useful for cases in which data is ajaxed after initial page load.
 - Forwarding of cache headers. In this mode, the service will inspect caching-related headers of the page that it is taking a screenshot of. When returning an HTTP response, the service will then set the headers to match. This mode only works when an image is being rasterised, not when an image is being retrieved from the internal file cache.
 - Clipping by selector. Clips the screenshot to an element defined by a CSS selector.