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 likely is it that the client library will find a newer version than what it supports? It's not like SVN is changing very often. ;-)

I definitely say an exception once the provided is selected is best, except in the example you gave where the library just can't handle it. It's odd that the client library could get thrown out of wack so easily. Do you run into this scenario? If not, maybe we just say don't use this plugin until svnkit supports you.