Revised means for fetching and storing PJSIP SVN revision number

darcs-hash:20090112152302-58103-0f227ed704f5ba11ac65842e5635b66a0022f215
Allow pjsip_svn_repo and pjsip_svn_revision options to build_ext module of setup.py to be overridden by PJSIP_SVN_REPO and PJSIP_SVN_REVISION environment variables respectively

darcs-hash:20090112154158-58103-bbadf705b45aa517fafe4c72f5848dd64676d970