Added fix for future UIWebView implementation and UIViewAutoresizingFlexibleHeight to _progressBarView autoresizingMask

Added fix for future UIWebView implementation. This has no effect at the moment because the NJKWebViewProgress does implement all UIWebView delegate methods. say there was a new method which NJKWebViewProgress did not implement. The UIWebView first checks to see if the NJKWebViewProgress implements this method and fails (it has no idea that the a _webViewProxyDelegate object exists!). So it won't forward the method to its _webViewProxyDelegate because we never check to see if the _webViewProxyDelegate responds to the selector. Fix is to override - (BOOL)respondsToSelector:(SEL)aSelector in NJKWebViewProgress and check if _webViewProxyDelegate respondsToSelector:aSelector .

Added UIViewAutoresizingFlexibleHeight to _progressBarView autoresizingMask. This ensures that it resizes with its NJKWebViewProgressView containerView. (When using constraints based layout the frame for NJKWebViewProgressView may initially be CGRectZero and the correct frame may be set afterwards, but the height of the of the _progressBarView would still be zero therefore invisible.)