Add vendor extensions to Content

Content gets vendor extensions (specification extensions) via a dict that can hold arbitrary additional properties.

Tons of additions, improvements, test coverage, and breaking changes.

⚠️ Breaking Changes ⚠️

• PathItem Parameter
• JSONReference
• JSONSchema

Fix annoying warnings and add integer expressibility to status codes.

• Fix annoying warnings from passing nil to string interpolation.
• Add integer expressibility for response status codes and add unit tests for Response file.

Make package available without OS X 10.13

OS X 10.13 is required for some conveniences I used for testing but the package works fine without it.

0.1.0

Merge branch 'master' of github.com:mattpolzin/OpenAPI