Add support for the libpq event system

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Original report by Michael Granger (Bitbucket: ged, GitHub: ged).

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libpq has an [[http://www.postgresql.org/docs/9.0/static/libpq-events.html | event system ]] that makes it possible "to notify registered event handlers about interesting libpq events, such as the creation or destruction of PGconn and PGresult objects". This might be useful for logging, connection pool management, etc.

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Original comment by Lars Kanis (Bitbucket: larskanis, GitHub: larskanis).

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I don't think, that it makes much sense in the ruby context.

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Original comment by Michael Granger (Bitbucket: ged, GitHub: ged).

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I agree. I think I was misunderstanding what exactly the events are used for when I opened this. Closing.