Adding identity samples

Partially addresses #422 
My original goal, when I first started adding these samples, was to make it dead simple for the user to *run* any of these samples. Specifically, I have been creating these samples such that the user has to perform the least number of changes possible to run them.

Lets consider this sample in it's current form (i.e. one sample showing all three methods of retrieving a user). It would take a user 3 edits to run it with, say, username:
1. replace `{username}` with the desired value, and
2. comment out the line using `{userId}`, and
3. comment out the line using `{email}`.

Contrast that with providing three distinct sample files. It would take the user only one edit to make one of those files (say the one that uses username) run:
1. replace `{username}` with the desired value

I know that sounds like a small delta but I think we should do whatever *we* can so the user doesn't have to. Also, what is the downside of having multiple files?

Another benefit I've found to having multiple sample files is that I can usually link to one of them without providing any additional explanation, when a user requires support for a particular use case of execution path for a service.