Add subvertpy as an alternative to PySVN

"subvertpy" is a new python library for subversion which has the distinct
advantage of being installable with easy_install. This change generalizes the
SVN SCMTool to use either subvertpy or PySVN, depending on which one is
installed.

Testing Done:
* Made sure there were no regressions in the unit testsuite (Python 2.7,
  Linux), and made sure that PySVN still works.
* Light manual testing (Python 2.7, Linux).

Reviewed at https://reviews.reviewboard.org/r/5104/