Add YAML spec parsing support

This adds support for parsing Swagger 2.0 YAML spec files by including [js-yaml](https://www.npmjs.com/package/js-yaml), and modifying *swagger-client.js* slightly to try parsing a spec as YAML after it fails to parse as JSON.

This PR is split into [two commits](https://github.com/swagger-api/swagger-ui/pull/782/commits), one with the code changes, and one with the *dist/* rebuild, in case they're useful to break up.