Propose to add Swagger doc support for Livy REST API

Current docs manually defines REST APIs, with the change of APIs the document may be obsolete, so we should have a program way to automatically generate REST API definitions.

Swagger (http://swagger.io/) is a powerful tools for REST APIs documentation, it has a UI to show and try REST APIs in a very convenient way (http://petstore.swagger.io/). Also Scalatra already supports generating Swagger definition (http://www.scalatra.org/2.2/guides/swagger.html). So here I'd like to propose to add Swagger doc support for Livy REST APIs.