Api update

[chris-smith]

Updates node handle sub, pub, service api to more closely resemble other roscpp/rospy. When advertising/subscribing, can supply either a string 'std_msgs/String' or the class from rosjs.require('std_msgs')

e.g.

const StringMsg = rosjs.require('std_msgs').msg.String;

rosjs.advertise('chatter', 'std_msgs/String');
rosjs.advertise('chatter', StringMsg);

Adds some basic pub sub tests. Separates out gennodejs tests from client library tests so that people don't need to pull test_msgs down in order to run the client tests.

[IanTheEngineer]

So this new api looks great, and likely what users will want. Can we support both the current and old API's in the future? That would make things slightly more flexible for users.