Allow use of time data when dealing with contributions via the API

Even though the fields are all datetime format in the database, time data is not allowed for receive_date, cancel_date, receipt_date, and thankyou_date when dealing with contributions via the API. Wikimedia needs the ability to record time data for our contributions, in addition to date data, due to the high volume of donations we receive. Previously, this was done via a custom API module. I have moved this functionality to the built-in API, but had to make two changes to get it to work:

1. Changed the data format check in the Contribution API to check for dateTime instead of date.
2. Fixed the dateTime check in Rule.php. (The previous version was not a valid check.)

This change is completely backwards compatible since dates are also valid datetimes. Patch for CiviCRM 3.2beta3 is attached.