Allow proper usage of namespaces for openni's nodes and nodelets

This is a slightly different patch for issue #2.
The core changes are:
* Nodelet manager runs on the same level as the nodelets, thereby removing the need for an absolute manager name as nodelet parameter. This allows pushing down all openni node's and nodelets in extra namespaces, e.g. for different robots, applications etc.
* Sorts the various nodelet topics (depth, ir, rgb, ...) by remapping the nodelets' i/o topics

Additional changes:
* The worker threads parameter has been made accessible.
* Assumed duplicated include of depth nodelets for depth_registered has been removed. This also makes the service registration error go away.