Remove NITE as a system dependency

This PR addresses issues #6 and #7 since NITE is technically not allowed to be repackaged anymore.  Openni_tracker does not need to be linked to NITE libraries, so having users manually post-install NITE works fine.  When NITE is not installed, an error message directs users to the README for download locations.  This should also allow release into Indigo which doesn't provide libopenni-nite-dev.