DO NOT MERGE: Issue 2106: Live Preview Snapshot

This is a PR for issue #2106.
`AKWebRenderer` uses this as a way to detect when a web view has *actually* finished loading. (i.e. redirects are done, dynamically loaded content is ready, etc).

The best example of this is when, if the website loaded has any redirects (JS, HTTP, etc), `webViewDidFinishLoad` would fire after each redirect has loaded, and, therefore, `webViewDidFinishLoad` wouldn't *actually* mean the web view has finished loading.

Unfortunately, I haven't found a bulletproof, framework-provided solution to more robustly accomplish this.

Fortunately, it looks like `WKWebView` in iOS 8 will exposes the private API which `UIWebView` uses internally for this (`estimatedProgress`), so we'll be able to more reliably determine this, in future releases.

Comment added.