enum34 dependency
Group A* arguments into a named tuple
Use PerceptronArgs tuple for arguments too
Continue astar arguments refactor

The hope is increased modularity in our arguments processing. Prior to
this strand of work, any option we wanted to introduce to A* would have
to be splattered throughout the code.

We still have to define them in a a couple of places:

* astar args
* shared state
* argument parsing

But maybe this will help us to get a better handle on it
attelo.args: pylint and pep8