Fix a bug where the `UIWebView` goes out of scope when a method calling `saveHtmlAsPdf:` or `saveUrlAsPdf:` finishes.

UIWebView objects were locally scoped to the method calls, meaning that when those methods returned the web view was deallocated. Now the webView object is scoped to the entire class, ensuring that the delegate methods are triggered and that the PDFs are generated properly. webView is disposed of after the callbacks, since web pages for PDFs may be pretty hefty in size.

Also updated the README with similar instructions for use: Your examples suggest that a local allocation of BNHtmlPdfKit will work as expected, however it goes out of scope before the delegate methods are called.