0.2.0

A bunch of changes, including some breaking changes (allowed before v1..):

• Removed Domain's Predicates property for now at least - I was a little overzealous in grabbing this from PDDL. Without some of the features that PDDL supports (e.g. a type system that needs to be validated against when e.g. referring to a predicate in an action), it doesn't serve much purpose
• Added a "domain" parameter to PlanFormatter ctor - ultimately empowering it to be able to print out an Action representation that just includes the identifier and what each of the variables from the action's schema are bound to.
• ElementDifferenceCount heuristic's method renamed to "EstimateCost". We don't have an interface here (because all we need is a delegate), but I'm going to standardise on this name
• Added a first pass at a heuristic that uses a set-covering, precondition-ignoring approach - mentioned in the source material.
• Added ToString overrides to State, Effect and Goal
• Minor tweaks to state space searches

0.1.0

It's very basic, but it can now create plans, so I'm publishing it. Like with SCFirstOrderLogic, not 100% convinced anyone will care, but.. meh, someone might find it interesting.