Add timerhdr field

Add support for the `timerhdr` field.

The `timerhdr` field is included in the example files at http://support.quickbooks.intuit.com/support/articles/HOW12778 (under QB 2007 & Enterprise 7.0 and later, at least). The documentation states:

> Data you export from or import to the QuickBooks Timer always begins with a header block. The keyword for the header is TIMERHDR. The header block can be followed by either activity data (TIMEACT) you are importing from the Timer, or list data (CUST, EMP, INVITEM, VEND, OTHERNAME, and CLASS) you are importing to the Timer.