Implement get item references

Hey,

I needed this method for a project I'm working on, and I couldn't find it in the ruby library, so I implemented it. https://developers.podio.com/doc/items/get-item-references-22439

I put the method in the Podio::Item class, and followed the preferred block syntax for the connection.  I also added a missing comment for the neighboring method :smile: 

Let me know if it looks good, or if I should change anything. 
In the meantime, I'm referencing my fork in my project.

Thanks!