Status of the DBI backend.

Code for all standard and business objects has been written.  The following
bugs/issues are known.

* Initial save performance is better than it used to be, but is still slow.
For example, saving my file with ~8000 transactions and ~21000 splits takes
13 minutes to an sqlite db.
* Foreign key definitions should be used for referential integrity.
* (Maybe) Keep an 'old values' table for each main table.  When a record is
updated, copy the old record to the 'old values' table first to keep track of
any changes.
* Use prepared statements
* Use cursor-based data models when reading from the db for better performance
* Write automated tests
* Write doxygen comment headers
* Don't need a log file
* Certain dialogs create an object when opened and update it as they go along.
This can result in trying to save invalid objects:
	- Price editor (new price has no commodity or currency)
	- New invoice
* Need transactions to group updates to multiple objects
* Possibly mark commodities that are in the db so that they don't need to be
queried every time.
* The sqlite db file is *much* larger than the gzipped XML file.