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).
Due to differences between git and svn, *head* here should probably be called *base* for clarity.  What we're getting is not really the head revision; it's the currently checked-out version of the working copy, which I believe is called *base* in svn land.