Add CycleError
Add tests for checking cycles
minor doc changes
add add_states tests
add test for _update_rule_for_states
add tests for _update_node_observed_status
correct _no_missing_states function and add tests for it
correct _no_extra_states function and add tests for it
add doc for add_states
Merge pull request #45 from ankurankan/dev

correct functions and add tests