Release 0.8.7

Release 0.8.7 is a minor release and contains bugfixes and new features:

• State configuration dictionaries passed to HierarchicalMachine can also use states as a keyword to define substates. If children and states are present, only children will be considered.
• Feature #500: HierarchicalMachine with custom separator now adds is_state partials for nested states (e.g. is_C.s3.a()) to models (thanks @alterscape)
• Bugfix #512: Use model_attribute consistently in AsyncMachine (thanks @thedrow)
• Testing now treats most warnings as errors (thanks @thedrow)
• As a consequence, pygraphviz.Agraph in diagrams_pygraphviz are now copied by transitions since AGraph.copy as of version 1.6 does not close temporary files appropriately
• HierarchicalMachine now checks whether state_cls, event_cls and transition_cls have been subclassed from nested base classes (e.g. NestedState) to prevent hard to debug inheritance errors