Merge branch 'feature/rework-referenceelement-interface' into 'master'

Add freestanding function referenceElement(geometry)

As suggested in #65, this merge request adds a freestanding function
`referenceElement(geometry)` that returns a reference element for a grid
geometry **by value**. This will work for all grids with geometries
based on the standard reference elements. For grids that want to return
something else, the behavior can be changed by providing an overload for
`referenceElement(const Geometry&, const GeometryImplementation&)` in an
appropriate namespace.

Moreover, it is also possible to get the type of the reference element
for `Geometry` using `Dune::ReferenceElement<Geometry>`.

Both the change to value semantics and
`Dune::ReferenceElement<Geometry>` rely on dune-geometry!52. I've
also tried to adapt all of dune-grid to the new interfaces (in
particular, the value semantics and the deprecation of
`Dune::ReferenceElement<ctype,dim>`) - but no guarantees.

I had to build in a pretty ugly hack to make the thing work on Debian 8,
which is contained in a single commit and should be reverted once we
drop support for GCC 4.9.

This merge request closes #65.

See merge request !194