Added support for lambdas

[esteve]

This is a quick hack to add support for lambdas.

Connects to #29

@dirk-thomas @tfoote @wjwwood

[esteve]

@dirk-thomas let me know what you think of this, it's not very readable, but works.

[dirk-thomas]

I updated the patch to match style. This works for me with the following userland code:

auto callback = \
  [](
    const std::shared_ptr<rmw_request_id_t> request_header,
    const std::shared_ptr<userland_msgs::AddTwoInts::Request> request,
    std::shared_ptr<userland_msgs::AddTwoInts::Response> response
  ) -> void
  {
    std::cout << "Got request #" << request_header->sequence_number << std::endl;
    response->sum = request->a + request->b;
  };

node->create_service<userland_msgs::AddTwoInts>("service_name", callback);

+1

[wjwwood]

+1

[tfoote]

+1