Fix issue #8, string intern error when starting state is an unknown action

This PR fixes https://github.com/rtwomey/stately/issues/8

Added a test for the scenario to state_spec.rb. The fix required the ACTION lookup to use `#fetch` with a default instead of the index.
```ruby
def guess_action_for(name)
  ACTIONS.fetch(name.to_sym, name)
end
```