Provide functions for C++11 range-based for loops. Document C++11 interaction.

This commit consists of the following individual commits:

Specifically, add Triangulation::cells(), Triangulation::active_cells() and
similar for the ones on a single level.

Fix up code to make it compile.

Fix one compile error.

Also provide similar functions for the two DoFHandler classes.

Augment documentation.

Update the specification of how we implement C++11 range-based for loops.

Link to the new documentation module (still coming up).

Document C++11 features and interaction.