Add basic support for Subversion by using export strategy.

Hi,

I've added some very basic code to support SVN as a SCM.  I am not a Ruby developer and have never written any Ruby code before until now, but hopefully this will inspire someone else with better (any!) Ruby knowledge to push on with the job of enabling SVN in Capistrano v3.  This commit goes in tandem with a Cap v3 tutorial series I am writing:

http://www.zodiacmedia.co.uk/blog/capistrano-3-tutorial-series

Thanks,

Billy