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.
Avoid setting the templated 'Number' to '0' (double) in trace(Tensor<2,dim,Number>).

Enables using trace also with e.g. Tensor<2,dim,VectorizedArray<double> >.
Add assertion for row index in TrilinosWrappers::SparseMatrix::set/add