Move sample lists to common.
Add THStack abstraction to common.
Make not having a suffix default.
Add a preliminary classifier plot script.
Add preliminary stacked plot script.
Ignore png and pdf files.
Update samples in common.py

* Remove TTJets_MassiveBinDECAY
* Separate ttbar
Update the preliminary stacked plot.
Add title and name parameters to ROC plot function.
Add a script to create ROCs for different backgrounds.