Add Subversion support and standardize source path separators

ScmInfoPluginProjectSpec assumes the source path components will be separated by /, which does not hold on Windows.  I left the test alone and translated the source path coming from the ScmProvider instead.

I added Subversion support, including an untested hook for the [Jenkins Subversion plugin](https://wiki.jenkins-ci.org/display/JENKINS/Subversion+Plugin).
How expensive is this query? We just have to be really careful since this is run every time Gradle is run.