Add a smart, lazy __all__

[brettcannon]

The idea being that maintaining all is annoying, so why not automate it?

It would need to be a part of __getattr__. That probably means implementing a fall_through argument so that you can stack __getattr__ functions (e.g. this with lazy_import().

[brettcannon]

Another option to a fall_through argument is to simply provide a generic AttributeError constructing function and then a helper function that simply chained together calls when AttributeError was raised. It would probably require setting an attribute attribute in order to help prevent accidental suppression of a different AttributeError.

[brettcannon]

Done in 182e3cc