ASIWebPageRequest fixes

ASIWebPageRequest excluded more multimedia formats from fetching ( for now: ogg,ogv,webm,mp4,avi,mpg,mpeg,mkv )

[ASIWebPageRequest addURLToFetch:] now doing percent escaping URLs strings
[ASIWebPageRequest contentForExternalURL:] now doing the same URL strings transforms matching these in [ASIWebPageRequest addURLToFetch:]