Update Gitian descriptors for Windows

This pull request is related to the issue #41.

With the updated descriptors, I was able to generate the binaries for Windows (32bit and 64bit), and the installers.
I tested the generated installer and binary on a Windows 7 32bit laptop.

The instructions to set up a Gitian builder environment are in the file "contrib/gitian-descriptors/README".
The list of dependencies to download and the commands to use gitian-builder are in the file "doc/release-process.txt".

To test the build process, you will need to change the 'url' field of the gitian descriptor "contrib/gitian-descriptors/gitian-win.yml" to make it point to a git repository containing the commit of this pull request.

Also, I will probably update the linux gitian descriptor too so that all the descriptors are based on the same library versions.