native conversion for more types

[stevengj]

Although PyCall converts objects to opaque wrapper types as a fallback, there is a long list of types for which a native conversion would be preferable:

• Julia Range <---> Python xrange
• Julia Set <---> Python set
• Julia composite types <---> Python classes
• Julia CalendarTime (from Calendar.jl) DateTime <---> Python datetime (via API)

and others.

[stevengj]

Closing this; it is better to have one issue per desired conversion.