If the function returns a future, handle things asynchronously

[spanezz]

Currently I can use @async to make a xmlrpc server method asynchronous, but with tornado.gen and friends, I tried to make it so that I could use a @gen.coroutine instead. The first step was to make it so that if an xmlrpc method returns a Future, it is run asynchronously and its result is returned transparently.

Unfortunately, @gen.coroutines still cannot be used transparently, because the wrapper that they create confuses tornadorpc's getcallargs. Still, I find this change quite handy.

With regards to using gen.coroutines, at least now I can do this:

class RpcHandler(XMLRPCHandler):
    def foo(self):
        @gen.coroutine
        def run():
            result1 = yield something(1, 2, 3)
            result2 = yield somethingelse(3, 4, 5)
            raise gen.Return(result1 + result2)
        return run()