Add example to Any documentation

Noticed there wasn't an awful lot of info out there on using Any types, so added an example to the rustdocs.
@alexcrichton Yeah, that's definitely a much better style - I guess my concern is that I kinda want to show off both is and as_ref functions.  I could perhaps rework the example to include a special case on a second type, where an if statement would be a more natural style.  What do you think?