Support for http downloads

I've just committed code to the website that finally allows http downloads. This has been a much requested feature for quite a long time, mainly from those users that are behind firewalls that don't work well with ftp. Most often a http download of a single file will also be faster than ftp download of the same file, since there are less roundtrips. Though for large files that difference shouldn't be noticeable.

The mirror checking script will poll the same mirrors we already have and figure out which ones support http download as well as ftp. Therefor, all mirrors listed will always support ftp, but some (currently about a third of them) will also support ftp. There will be no http-only mirrors.