Added noTime TimeType support

The Palm library now properly supports noTime constants in Palm tables. I've also added an IsNull property and Clear methods to the field class to make it easier to check for null values.

Note: a side-effect of adding TimeType null support is that existing date values can no longer default to a "legitimate" value (particularly since null times are actually a valid 12AM time). So when reading a Palm table with a date or time field that can be null, your code should explicitly check to see whether it's null or not.