Support for date/datetime parsing functions

[vruusmann]

The PMML specification supports temporal data types:
http://dmg.org/pmml/v4-3/DataDictionary.html

It should be possible to parse strings into "proper" date/datetime objects using the base::strptime function, and then do basic arithmetic on them (eg. calculating deltas).

For example, it should be possible to do something like that:

as.formula("y ~ I(strptime(x1) - strptime(x2))")