Consider adding fetch to eventbrite_object.rb

First of all, great gem

Was wondering if you had considered adding a method to  eventbrite_object api that would delegate to Hash#fetch? 

This could be handly if I do something like:

```
if eventbrite_event.fetch('logo', {})[:url].present?
...
end
```

http://ruby-doc.org/core-2.2.0/Hash.html#method-i-fetch
