Fixes #227 (Maybe!) Updated Gtest to r643 (Feb 28)

- Added a shell script (test/gtest/update.sh) to simplify
  updating/selecting an appropriate gtest revision

- Used this script to update to the latest version of gtest (r643).

- Updated shell script to die if SVN not found in PATH

- .gitattributes now converts *.cc files to unix line-endings
  (*.cc text eol=lf)

- Added license header to test/gtest/update.sh