Add subvertpy support, in addition to the existing PySVN support. This allows SVN users to use virtualenv + pip to install Review Board and its dependencies. If subvertpy is not installed, it falls back to PySVN.
Made the SVN implementation modular, so other libraries may be implemented if desired.

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).