Add find_entities helper method, for easy conversion between name and ID

This is the function that I'm having to define in each piece of code that uses flapjack diner 1.0.

@ali-graham What are your thoughts on including this into diner before 2.0 comes out?  
I'd rather see that be an `eql?` (if `name` there is a string) or `name.match...` if name is a Regexp.