Add support for shared models

[jwyuen]

Basically #2 in this link:

swagger-api/swagger-js#35 (comment)

This would help keep api docs DRY.

[mission-liao]

+1, I think it would be a better choice of spec-design.

[webron]

Swagger 2.0 is going to have a slightly different file structure. Since everything could be represented in a single file, it would eliminate the need for repeated model definitions.

[webron]

This indeed has been resolved in Swagger 2.0. The specification is now theoretically a single file so all the models are defined in a single place. Furthermore, you can $ref to an external file and share models across swagger endpoints.