Change DBO.php to use an ORM

[franc6]

Using a real ORM is probably desirable, instead of hand-coding SQL. This would also simplify unit tests for DBO.php, since it wouldn't need to use a real database, insert data, and verify the data coming out is what's expected. Instead, the unit tests could simply confirm that each method in DBO invokes the expected ORM methods.

I don't know much about ORMs in PHP, so I don't know which would be a good fit. Doctrine and Propel seem to be the most popular.