Allow YAML anchors

[ehmicky]

YAML anchors like &anchor and *anchor on objects or arrays throw the following error:

.../node_modules/recursive-iterator/dist/recursive-iterator.js:129
	                                throw new Error("Circular reference");
	                                ^

Error: Circular reference
    at RecursiveIterator.next (.../node_modules/recursive-iterator/dist/recursive-iterator.js:129:40)
    at seekWrong (.../node_modules/swagger-jsdoc/lib/swagger-helpers.js:230:64)
    at .../node_modules/swagger-jsdoc/lib/swagger-helpers.js:259:5
    at Array.forEach (<anonymous>)
    at Object.findDeprecated (.../node_modules/swagger-jsdoc/lib/swagger-helpers.js:257:11)
    at module.exports (.../node_modules/swagger-jsdoc/lib/index.js:108:35)
    at createSpecification (.../node_modules/swagger-jsdoc/bin/swagger-jsdoc.js:44:34)
    at .../node_modules/swagger-jsdoc/bin/swagger-jsdoc.js:191:5
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:525:3)
.../node_modules/recursive-iterator/dist/recursive-iterator.js:129

The reason is when using YAML anchors, references are created. However there seems to be a bug in recursive-iterator where if two siblings are the same reference, this still throws a Circular reference error.

I fixed it by allowing circular references in recursive-iterator.

[kalinchernev]

@ehmicky I merged #109 few minutes ago and there won't be anchors in the specification object any more. Thus, I think there won't be any more cases of circular dependencies, right? (if users don't overuse the swagger $refs :)

[ehmicky]

Correct me if I'm wrong but I think #109 was about YAML anchors when swagger-jsdoc serializes the JavaScript object to YAML. This is about references due to YAML anchors when swagger-jsdoc parses YAML to a JavaScript object, i.e. is probably still relevant.