freshing up for release

adding a Licence: GPLv3

changing the README into the md format

adding a copyright banner to GammaComboEngine.cpp

improving the usage output
Merge branch 'master' of https://github.com/mkarbach/gammacombo
changing md syntax
adding download instructions
adding instructions to run tutorial
improved group label alignment
Add an additional control plot for plugin method which shows the chi2
p-value distribution of the toys
Merge pull request #3 from matthewkenzie/PluginControlPlots

Add an additional control plot for plugin method