Downloads for website via a repository
Instead of using the 'Downloads' part of bitbucket for downloads, it makes sense to use a repository instead.
This way syncing with the website can be done more easily, and it also makes it easier for the buildbots. The 'Downloads' section does not have an API currently, which makes dealing with them hard with scripts.
Also, some minor points are that this allows people to submit downloads via a pull requests, and it also provides a history and MD5 sum for the downloads.
- create new 'downloads' repo under the 'pygame' team.
- move all old downloads into this new 'downloads' repo.
- update release documentation, and hacking documentation to reflect this.
- website script which generates downloads webpage based on update information from hg 'downloads' repo.