Get content from SVN if no local checkout

Replace PYTHON_ORG_CONTENT_SVN_PATH with commented-out empty string.
Add PYTHON_ORG_CONTENT_SVN_URL variable. Defaults to beta.python.org SVN
If no local path exists, default action is to checkout to /tmp and build

Fixes #369

I'm sure this breaks on Windows. Someone with Windows experience could improve it by adding a similar temporary directory there.