Added initial "identifiers" section

I've added an initial identifiers section, based on http://groups.geni.net/geni/wiki/GeniApiIdentifiers

This needs some work. In particular, I've used the placeholder ZZZZ instead of geni, because I am unsure what exactly we need to put here. Suggestions are welcome!

Note that this already includes changeset O1 from google doc issue 3 (as this was in fact already added to the AMv3 API!).