Proposal: Initial namespaces proposal

This is based on feedback and discussion that came out of #1017 

There was discussion that the term project may be problematic, so have removed it in favor of namespace.

I think this aligns nicely with the ACL model discussed thus far.

Feedback appreciated.

@erictune @lavalamp @smarterclayton 


> An ID is unique to a Namespace.

The terminology in identifiers.md is "identifier" - so "identifier is unique to a namespace" is more accurate.  The value the rest API is exposing for ID correlates to an "identifier" in all our cases today.  The RFC universally unique identifier is not what the ID field is today (because users can specify it) so it would be a different attribute - whether that's uuid, uid, guid, creationid, whatever, can be discussed in the other issue.