Support model property descriptions

[bherbst]

This PR adds support for annotating model properties to provide a description (see #52).

Sample usage:

@Response("A String Response")
data class StringResponse(
    @Description("The string value") val str: String
)

Generated Swagger JSON:

 "StringResponse" : {
        "nullable" : false,
        "properties" : {
          "str" : {
            "description" : "The string value",
            "nullable" : false,
            "type" : "string"
          }
        },
        "required" : [ "str" ],
        "type" : "object"
      }

Swagger UI: