Enum to string (for #317)

`Catch::toString` support for `enum class` types.

This is pretty straightforward - if we get as far as `StringMakerBase<T>`, then we check to see if `is_enum<T>`, and if it is, we call `Catch::toString` with `underlying_type<T>`.

Tests are in EnumToString.cpp

(NB: I have *not* rebuilt the `single_include` header).