Simplify architecture.

mv AbstractLayer Layer
mv Layer/*/*Statement.php Layer/*/Statement.php
Link \PDOException as a previous exception.
Add the hoa://Event/Database/<id>:opened event.

And add the getId() method.
Add the close() method and :closed event.
[auto] Merge from Database

    • #ba298aa — Simplify architecture. (Ivan Enderlin, 2013-08-10)
    • #19c54b2 — Link \PDOException as a previous exception. (Ivan Enderlin, 2013-08-10)
    • #e3e5924 — Add the hoa://Event/Database/<id>:opened event. (Ivan Enderlin, 2013-08-10)
    • #ef7b546 — Add the close() method and :closed event. (Ivan Enderlin, 2013-08-10)

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Merge remote branch 'library-database/master'