Implement support for Python operators

[miggol]

Implementing __lt__, __gt__, and __contains__, we could more easily figure out if a person with a certain date of birth is contained within an age range

[bbonf]

I've added __contains__ in c3af741
I don't mind adding < and > but I'm not sure how to combine them with the idea that an age predicate represents a range already.

[miggol]

I've added contains in c3af741

Nice! That's probably the most important one anyway.