refactor modifications to swagger 2.0 schema

To see the actual diff between the original swagger 2.0 schema doc and this latest version, look at this diff:

https://github.com/hornc/apivore/compare/5bfb7c91b10c3ad70e8b4eaa9f135d0f8c8d1016...de95d9519eac348ed3867e7fe5a86c4a0b19b22b#diff-7

The first set of changes I made just pasted the definitions from the external document in (which involved duplication). This makes use of internal refs in a more sensible fashion.
The definition of minItems in the defs block is a $ref to positiveIntegerDefault0 :)

Now it's only defined in one place.