72x 20140815 weight

The major changes include:
1) Added a class GenLumiInfoProduct to save cross section in each luminosity block
2) GenFilterInfo: add sum of weights and weight-square, number of passing and failing events with positive and negative weights
3) Updated LHERunInfo, GeneratorFilter and HadronizerFilter so to save GenLumiInfoProduct in each luminosity block for both ME generator and parton shower generators
4) Added an analyzer GenXSecAnalyzer to combine the cross sections from each luminosity block and compute the cross sections before and after generator filters.