Allow PDOStatement objects to be passed in (to allow mocking)
Added tests for pre-execution methods
Switched to dependency injection
Moved set_pdoStatement() to parent object
Flushed out bindValue() tests
Flushed out bindParam() tests
Added error-checking for when pdoStatement isn't set
Added tests for when PDOStatement wasn't set on the dbStatement object
Missed adding the last test to execute() as well