adding some functions to resolve ros message type

[garaemon]

1. Ros.getTopicType
   A wrapper of /rosapi/topic_type
2. Ros.getMessageDetails
   A wrapper of /rosapi/message_details
3. Ros.decodeTypeDefs: adding a utility function to resoluve ros message definition
   The function convert the result of getMessageDetail into a dictionary.
   For example, sensor_msgs/JointState will be converted like:

{
  "header": {
    "seq": "uint32",
    "stamp": "time",
    "frame_id": "string"
  },
  "name": [
    "string"
  ],
  "position": [
    "float64"
  ],
  "velocity": [
    "float64"
  ],
  "effort": [
    "float64"
  ]
}

[rctoris]

Hi @garaemon -- great start! These are indeed so very useful functions. There a few convention, style, and minor bugs in the file which is also preventing it from properly running through our Grunt build (Travis-CI build output here: https://travis-ci.org/RobotWebTools/roslibjs/builds/12611739). We have listed our guide here as well: http://wiki.ros.org/JavaScriptStyleGuide -- building locally can also be found here: https://github.com/RobotWebTools/roslibjs/tree/devel/utils

I'll add some comments where I see some errors in the pull (also note it will be looking for 2-space indentation).

[rctoris]

You are never using this local variable.

[garaemon]

I've fixed the points you mentioned.