Restructure SVN support work.

An updated version of https://github.com/capistrano/capistrano/pull/838. I don't really see a benefit to checking out a local copy of a project from SVN as it is not used in the deployment process.  As the class Capistrano::SCM class mandates the implementation of the methods test, clone and update, which all relate to a locally maintained repo I have just made these methods return true.