Add convenience JSON/NSDictionary factory method to OHHTTPStubsResponse

This new method `responseWithJsonObject:statusCode:responseTime:headers:`
allows easy construction of JSON-based stub responses.

It accepts an NSDictionary, or anything accepted by
`+[NSJSONSerialization dataWithJSONObject:options:error:]`, and returns
that object as JSON in the response body.

This makes ad-hoc definition of lightweight responses inside unit
tests simple and easy.