Accept download web pages in addition to direct download URLs

[homebysix]

Example of how this enhancement might work:

1. You'd input a download webpage to Recipe Robot (e.g. http://www.netspotapp.com/downloads.html)
2. Recipe Robot would parse that page for automatic downloads (e.g. http://www.netspotapp.com/download/NetSpot.dmg), and if one exists, use that as the download_url for subsequent inspection.
3. If no automatic download exists, Recipe Robot would parse the page for zip, pkg, and dmg links, and use that as the download_url for subsequent inspection.
4. Recipes would be created that leverage the derived download_url.

Thanks to @nmcspadden for the suggestion.