Paypal payment callbacks

This could be done:
```ruby
@callbacks = callbacks.map { |(ev, cbs)| [ev, Array(cbs)] }.to_h
```
thus making the ensure_ary unnecessary. 

http://devdocs.io/ruby/kernel#method-i-Array
(or using, but let's not go there http://devdocs.io/rails/array#method-c-wrap)