Catch::toString support for std::tuple.

Obviously, this only works with C++11 or later enabled.

Perhaps worth noting that (last time I looked), Travis' versions of `gcc` and `clang` are lacking decent C++11 support - so none of these tests will run there.

Pass `-DUSE_CPP11=YES` to `cmake` to enable the C++11 tests. (tested here w/ gcc4.8.2, clang 3.5 and clang 3.6, each in C++11 and C++03 modes)