Add a plugin to account for lepton scale factors

The file with muon scale factors was prepared with the help of code in
[1], which converts scale factors from [2] into a more convenient
format.

[1] /afs/cern.ch/user/a/aapopov/workspace/tHq/2012Bravo/2014.05.27_Muon-scale-factors
[2] https://twiki.cern.ch/twiki/bin/viewauth/CMS/MuonReferenceEffs#22Jan2013_ReReco_of_2012_data_re
Add a file with electron scale factors

The scale factors are extracted from tables 11 and 12 in AN-12-429 [1]
[1] http://cms.cern.ch/iCMS/jsp/db_notes/noteInfo.jsp?cmsnoteid=CMS%20AN-2012/429