Ignores $db_prefix

Timesheet.module ignores $db_prefix and uses MySQL specific DDL.

The attached patch:

1. provides a translation of timesheet.mysql in timesheet.pgsql
2. handles PostgreSQL correctly in timesheet.install
3. fixes all db_query() calls in timesheet.install to honour $db_prefix
4. fixes the misnamed column hl_form in {timesheet_holidays}