Allow $refs in Components Object Maps

[MikeRalphson]

Now this may be controversial, but I believe this small added complexity has several benefits in writing and structuring OAS definitions:

• Allows aliasing of reused components, i.e where they are semantically different, but syntactically the same
• Allows easy extraction of a reused component to an external reference, without having to update all the source $ref pointers
• Gives us future expansion possibilities, such as proposed JSON schema merge keyword or RAML-style type extensions
• It will make converting existing 2.0 definitions easier, as the 2.0 Schema Object had a $ref property
• Due to the prevalent tooling'$ref anywhere' problem, we prevent ambiguity/abuse of the specification by explicitly allowing this construct

[RobDolinMS]

Tagging @fehguy @darrelmiller and @webron for their two cents

[darrelmiller]

Ron and discussed a similar idea that I had suggested last week when we were trying to get RC1 done. But I had added the reference object one level higher and it caused issues that I didn't like so we bailed.
This is a interesting compromise. It does require explicitly referencing every element that you are re-using from an external source, but that ensures that $refs in the spec itself could be kept just local.

In general I like this idea. It is simple to describe and fairly simple to implement.

[RobDolinMS]

TDC: @webron, @fehguy, and @darrelmiller give this a thumbs-up