fixes for ParticleReplacer

 - call beginRun with edm::Run const& instead of edm::Run &, as the
interface has changed in CMSSW 6_X_Y somewhere
 - initialise tauola with the random number generator in the constructor,
i.e. before the main initialisation in beginRun(), otherwise we get an
error here.
 - Make sure the check from embedded-gen Z pT actually does what it's
supposed to do