Implement rosnode API

[gelin]

In Python we have very good methods in rosnode package: http://docs.ros.org/hydro/api/rosnode/html/
Technically these are XMLRPC calls to http://wiki.ros.org/ROS/Master_API and http://wiki.ros.org/ROS/Slave_API

In rosgo XMLRPC connection to ROS master is hidden.
It'll be very useful to have a public wrapper over it repeating methods of Python's rosnode package.