Eliminated objects that hold constant strings. Aids step-debugging in MSVC

This is a re-submission of the second half of my previous pull request https://github.com/philsquared/Catch/pull/203

I've eliminated std::string objects that hold constant strings, and reverted to char const*. This helps step-debugging in MSVC where "step-into" a CHECK (for example) steps into several ctors and dtor for creation/destruction of temporaries.