add can_execute_event? to check against guard clauses

See issue: https://github.com/troessner/transitions/issues/127

I would personally go further and just replace the implementation of ```can_event?``` with ```can_execute_event?``` entirely, but I suppose some people will have scenarios where that would be a breaking change. 
Couldn't we use 

```Ruby
can_#{name.to_s}?
```

here?