Observer 2.0.1

pg::connection_owner now uses a std::vector instead of a std::set to store connections.
Added pg::connect functions which return a pg::scoped_connection.
Renamed pg::observer_owner to pg::connection_owner and provided an alias for the old name to ensure backwards compatibility.
pg::connection_owner supports static polymorphism for subjects.
Added pg::blockable_subject.

The overload pg::connection_owner::connect which connected one subject to another is removed. Use the connect overload for member functions instead. See tests or chain_subjects example.
pg::subject is no longer blockable, use pg::blockable_subject instead for cases that needs to block a subject.
pg::subject_blocker now requires subjects to provide a block and a unblock function, for example like pg::blockable_subject.
A connection is no longer automaticly invalidated. The usability of this feature didn't justify the extra complexity.

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