Add 'rvalue reference for *this' overload for core::optional<T>::value_or

Remove all warnings exhibited by clang and gcc

An upgrade to gcc 4.8.1 is required to use MNMLSTC Core now. This will
be reflected in the documentation, and in the Readme shortly.
Update requirements section of Readme
Add 'rvalue reference for *this' overload for core::expected<T>::value_or

Add [[noreturn]] attribute to core::expected<T>::raise to silence
warnings in clang and gcc.

Add small test for core::expected<T>. This test isn't fully flushed out
and will still fail explicitly.
Add test for core::optional<T>::value_or to extend coverage
Update internals of core::expected<void>

Mark core::expected<void>::raise as [[noreturn]]
Add test for core::expected<T>::operator * ()