Pledges with YEAR frequency unit sets wrong due dates for pledge payments

If you enter a pledge with frequency = 1 year, we still use the frequency_day field to calculate payment due dates. This doesn't make sense because you can't set that value to anything > 31 (it's intended for monthly and weekly frequency units).

For Annual pledges, we should disable the frequency_day field and use the start_date field to set pledge payment dates. EXAMPLE: If frequency is every 1 year and start_date = November 25, 2010, then the next payment date = November 25, 2011, etc.