Add convenience macros for enabling Variant <-> enum conversions.

[ecorm]

Something like:

#define CPPWAMP_ENUM_CONVERSION(TEnum_) \
void convert(wamp::FromVariantConverter& conv, TEnum_& e) \
{ \
    using IntegerType = std::underlying_type<TEnum_>::type; \
    e = static_cast<TEnum_>(conv.variant().to<IntegerType>()); \
} \
\
void convert(wamp::ToVariantConverter& conv, TEnum_& e) \
{ \
    using IntegerType = std::underlying_type<TEnum_>::type; \
    conv(static_cast<IntegerType>(e)); \
} \

[ecorm]

Instead, make it so that CppWAMP converts enumerations to/from integers automatically, while allowing the user to specify a custom convert function for their enum types.