0.7.2 Bump
Add Transition.message field

This enables adding more detailed information about the transition
such as a description, fail message, etc.
Add support for negative integers in System.__getitem__

This was missed in the initial implementation.
Add `current_state` and `in_transition` methods to STSModel

Additionally, the system object is exposed as a property for direct
access.
Change context manager to be class based with better exception handling