Consider harmonizing processor interfaces

[goodmami]

The ACE and REST interfaces do not have the same methods, so they cannot be easily swapped as processors in some pipeline (see #127). A standardized interface like this might be useful:

class BaseInterface(object):
    def process_item(self, datum):
        ...

It would also be nice if they harmonized on run information, although it makes somewhat less sense in the REST interface.

There's also a question of how to deal with parsing vs transfer vs generation. For ACE, the subclass model works well, but that doesn't work for the REST client.

Any breaking changes should wait retain backward compatibility until v1.0.0, and those changes should be listed in #139.