[JENKINS-777] Add ability to set depth and ignore externals for a module

This change is based on the same code from the hudson subversion plugin, see commit https://github.com/hudson-plugins/subversion-plugin/commit/b2ac2f82a38c7ea0537554dc0eb9c73c9a459411

I've added a coupleof unit tests and made the code work in the jenkins subversion plugin.

I assume the code is pretty robust as it has been in the hudson plugin for over a year.