primary vertex trimmer (HLT)

new module PixelVertexCollectionTrimmer has been added
in order to trim the primary vertex collection (HLT porpoises)

moreover, a config file for setting the PVClusterComparer PSet has been added
w/ the idea of exploiting the new refToPSet_ feature
and try to have a coherent setting among different customers of the PVClusterComparer
and --above all-- to guarantee the consistency w/in the PV producer between the track pt min threshold 

@martin @perrotta @cerati @rovere @innocente