Create a soft_def shim to combine move_to and Getattr

[anivegesana]

New (hopefully more intuitive) construct to combine move_to and Getattr into one function call.

Resolves #447

[mmckerns]

This looks good, in general. I don't like the name soft_def. You aren't actually "defining" the shim, you kind of do that above when you write the function code... it's more like we register the shim in _dill. @anivegesana: What do you think of the name register, or something similar? Otherwise LGTM, and will pull when we agree on a name.

[anivegesana]

register is already taken by the register function for the pickler. What about if_not_defined_in or ifndef_in?

[mmckerns]

How about: register_shim? Isn't that what you are doing? You are registering the shim in _dill (and setting a default). Also... one other thought... is it worthwhile to enable passing the module as a keyword arg (with _dill as the default)?