3rdparty: using tar file instead of SVN checkout for GMock

Sometimes the SVN checkout fails due to unknown reasons, thus it seems
better to directly download the tar file of the googlemock sources.
Merge pull request #53 from torbjoernk/feature/gmock-download

3rdparty: using tar file instead of SVN checkout for GMock
Merge branch 'development' into feature/mpi-pf