Spike - encapsulate Segments API into an overridable class #51

Currently everything about dpath's 2.0 behavior is wrapped up in the segments library, or virtually all of it. We'd like to be able to easily specify whether the v1 or v2 API is being used. We could easily break this out into "v1" and "v2" submodules, but that would break backwards compatibility for 1.x users who expect everything to be in dpath.utils (and, frankly, it gets in the way of dpath's simplicity - ultimately this is a utility library of only a few methods).

Let's see how it looks when we take the segments API and move it into some kind of "DPathBehavior" class, which is (by default) 2.0 behavior. Then let's see how easy that is to subclass to v1.0 and make the API accept a behavior implementation class, instead of a ton of flags.