make CombineHarvester code visible everywhere in CMSSW

[ajgilbert]

can do something like:

    <Flags CXXFLAGS="-Isrc/HiggsAnalysis/HiggsToTauTau/CombineHarvester" />
<Flags LDFLAGS="-Lsrc/HiggsAnalysis/HiggsToTauTau/CombineHarvester/CombineTools/lib -lCHCombineTools" />

in the BuildFile

[ajgilbert]

Will be done by switching to a new repo with CH as a top-level CMSSW package. Main tasks:

• Compiling C++ to python lib needs a few extra bits in the BuildFile
• Making the python lib visible can be done within an __init__.py that loads the lib file explicitly from $CMSSW_BASE/lib
• Still want to generate the GitVersion.h file automatically. Can execute a small makefile from the BuildFile.xml, but needs testing
• Have to move the documentation and make sure it's building ok
• Try to preserve the git history of the code development. Not essential but would be nice.