Release 0.5.2

The new release adds some reporting features, and a couple of bug fixes, but it's quite important to upgrade to this release - 0.5.1 (and possible 0.5) introduced a flawed piece of logic relating to issuing invoices; the 'number' field in the 'invoice_items' table was changed to a FLOAT. This is not a good idea and so it is back (for ever) to being an INT.