Merge branch 'release/0.27.0'
Update DOI for 0.27.0
Merge branch 'release/0.28.0'
Merge branch 'master' of github.com:EducationalTestingService/skll
Update DOI for 0.28.0
Add data package modules to skll.rst
Update copyright year in documentation
Add contribution guidelines, mostly taken from scikit-learn.
Merge branch 'master' into develop