The get_topic_data function in nodes/api.py isn't general nor robust.

[goruck]

The get_topic_data function in nodes/api.py does not work except for some specific topics and will hang if a topic isn't publishing messages. I rewrote it to be general and with better error handling. See PR #339 for those code changes.

Note that this api is very useful if a user wants a current message from a topic since the data comes directly from ROS and not from CouchDB. Its also faster since getting a current view from CouchDB is very slow.