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. 
There is no official style guide for this gem, but we're trying to follow https://github.com/bbatsov/ruby-style-guide so let's use brackets for method signatures.