new SVN step does not have a mode that eliminates .svn dirs

[dustin]

The old step's mode="export" produces a source dir without the distracting svn metadata, which can be useful in cases such as making tarballs.

We should add a new method for mode="full" that does an export.

[Dustin J. Mitchell]

Merge branch 'svn_export' of  git://github.com/in3xes/buildbot

• 'svn_export' of  git://github.com/in3xes/buildbot: Add export method to svn. Fixes #2078

Conflicts:

master/buildbot/steps/source/svn.py

Forward-ported (by dustin) and altered to not assume workdir='build', Release Notes added, docs edited.

Changeset: e82868da0232622fffee7b8408b51f4c0409850d