[quad] reanimate support for quad precision arithmetic
Add CMake detection by copying FindQuadmath.cmake from
opm-common/cmake/Modules at https://github.com/OPM/opm-common/,
version 37da204.
Remove hack for GCC <= 4.6. Add Dune's IsNumber specialization.
Test by using quad as Scalar and recalculating the one-phase
incompressible TPFA test with analytic Jacobian.
A quite annoying detail is that dumux/common/quad.hh has to be included
before any other Dune module, at least I couldn't find a better way.