Improve init-repository.py to fetch up-to-date dependency list

This patch introduces a new mechanism to retrieve submodules.
The utilities in version_resolver.py provide means to download
the upstream DEPS file for a specified Chromium version.
This makes it possible to check out the needed svn branches
and specific revisions used for the release. We still use the
git repositories found in the upstream .DEPS.git file but since
it is not regularly updated after the branch-off, we try to match
the release svn revisions to git shasums which is possible since
each commit includes git-svn-id's.
Also add a maintenance tool get_version.py which reads the Chrome
release information from http://omahaproxy.appspot.com.

We now pin the latest stable channel Chromium release from branch
1650 with the version number 31.0.1650.63 and a branch-off date of
Sept 23rd.

Change-Id: Ibbf565f876af79dc008bcd161fddcbdd8bc73977
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>