Add release signatures link on the download page

This pull request adds a "Verify release signatures" link on the download page pointing to : http://sourceforge.net/projects/bitcoin/files/Bitcoin/bitcoin-{{site.DOWNLOAD_VERSION}}/SHA256SUMS.asc/view

Screenshot:
![capture du 2013-09-12 22 51 36](https://f.cloud.github.com/assets/3578089/1135700/38e5f3e0-1c20-11e3-98d8-ea86bc5218ed.png)