Add informative __repr__ methods

[kgbaird]

This library would be much more user-friendly if the custom objects included informative __repr__ methods on them.

[p2]

Yes sure! What would you like to see in there? I often use as_json() but am guessing it's overkill to add that to the repr.

[kgbaird]

In our use, it appears most of the objects are just key-values of attributes. Having a visual of the attributes would be nice. as_json() doesn't sound that bad to me, but it probably is a bit much.