Update Gitian descriptors for Windows and Linux.

This pull request is related to issue #121.

With the updated descriptors, I was able to generate the binaries for Linux (32bit and 64bit), the binaries for Windows (32bit and 64bit) and the installers.

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 can pass the --url argument to gbuild in order to use a git repository containing the commits of this pull request.