Jet probability calibration rerun

Instruction to rerun jet probability calibration in simulated events as recommended in [1] have been added. Data from 22Jan2013 rereco is fine.
[1] https://twiki.cern.ch/twiki/bin/viewauth/CMS/BtagPOG#2012_Data_and_MC_EPS13_prescript
C++ source files moved to plugins subdirectory

The subpackage declares plugins only and does not provide a library to be used in other CMSSW subpackages. Therefore, it is more reasonable to keep everything in plugins.
Build files have been adapted to the new subpackage organisation

Documentation for the build files can be found in [1]. Paths in #include statements have been updated to point to the correct locations.
[1] https://twiki.cern.ch/twiki/bin/view/CMSPublic/WorkBookBuildFilesIntro
Wrong name for subpackage library has been corrected
Dependencies that are not needed have been removed from build file