Improved supported data-types in min() and max()

When I started only numeric types (int and float) were supported.
After my patches 'Date' and 'String' work as well (I added a JUnit test to prove this).

Please carefully review this as I'm not sure if my new "aggregate" function in 1504b6fec20c6472b8b7486c8cd38c4ecec14330 is good the way it is (and I'm not sure if this should be public API/has to be documented?).

PS: I only tested sqlite so far.