Capturing CanonicalUrl in Page

- Page#getWebUrl gives you either canonical url (if present) else page fetched url
- Page#getFetchedUrl will always give you the fetched page url
- HtmlParseData contains canonicalUrl field

TODO - According to Google Web Master Tools (http://goo.gl/HI6b5Y) if rel=canonical
is not equal to the current page, they do try to fetch that page. I am not sure if
we should need to do that?