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?  
Maybe just

```ruby
def entities_matching(name_re)
  raise "Must be a regexp: #{name_re.inspect}" unless name_re.is_a?(Regexp)
  self.entities.select {|e| name_re === e[:name] }
end
```

? Then you could get the `start_with` behaviour with, e.g. `entities_matching(/^db-app-01/)`, but aren't sacrificing flexibility.