Replace all occurrences of std_cxx1x by std_cxx11.

Keep the old files and namespace for backward compatibility.
Replace all occurrences of std_cxx1x by std_cxx11 also in tests/.
Merge pull request #111 from bangerth/replace-std_cxx1x-by-std_cxx11

Replace all occurrences of std_cxx1x by std_cxx11.