Added exceptional class which is implicitly convertible to expected.
The idea is to replace the exceptional_tag and rename it exceptional.

Added some overloads to then to unwrap implicitly expected<expected<T>>.

Added a get_exceptional function returning exceptionals<E>. Useful when
the expected is invalid and the result needs un implicit constructor
from the exceptional value.

Fixed some issues with comparaision of expected<void>.

Added safe_divide example, which could be used as a simple example of
the proposal.

Added jamfile on test directory.