Import Swagger YAML files.

[fabiocognigni]

The Swagger Java parser accepts both YAML and JSON files content.
This addresses #585

Notes:

• tried https://gist.github.com/earth2marsh/264123aefc2e29551d9d but it doesn't seem a good spec (note all $ref doesn't have #/defintions/ prefix);
• this seems to fix it but still apidoc can't import it: I can reproduce the same error with the same spec in JSON format (translated using swaggerhub) so I believe the issue doesn't depend on these changes but it's more deep in the swagger integration regardless of the swagger format - troubleshooting was taking too long so I'll probably open a new issue for this;
• tests with YAML versions of JSON specs that were correctly imported looks fine.

[mbryzek]

lgtm

[fabiocognigni]

@mbryzek this is now completed

[mbryzek]

nice! merging

[mbryzek]

now live