User defined server roles looks interesting. Thanks for the question Steve.
A minor correction is needed on the reference. It points to fixed server-level roles not user defined and includes earlier versions of SQL Server.
The correct documentation should be: http://technet.microsoft.com/en-us/library/ee677610.aspx
Interestingly, I could not find a reference for 2014 so the correct answer may be 2012 instead of 2012 and above. More likely though is that the 2014 documentation isn't complete yet.
Edit: It took a little longer but here's the reference for 2014 and 2012.