Fixes for TF and remappings

#8: TF caching is implemented [as proposed](https://github.com/srv/viso2/issues/8#issuecomment-15403249) by Stephan.

#9: The possibilities to remap a stereo namespace or image topics are gone because this functionality didn't work as expected. Instead, the rather standard concept of [pushing down](http://www.ros.org/wiki/Nodes#Remapping_Arguments.A.22Pushing_Down.22) is recommended. Also please note that the new topic API is compatible with [another VO engines](http://www.ros.org/wiki/vslam_system/Tutorials/RunningVslamOnStereoData#Necessary_Data).