updating fastjet contrib package for subjets

Hi Christoph,

  this is what is needed to update the fast jet package in order to have access to subjets.
I have taken JT latest nsubjettiness and created a new fast jet contrib tar ball which is
now available using my T3 web area. This new tar ball is used by default in the install 
script.
I have also added a simple script to make sure symlinks to the compiled external libraries
are properly updated, so you can start from a pre-existing CMSSW release.

best
Leonardo