Release version 0.80 (dmlc#3541)

* Up versions

* Write release note for 0.80

Release version 0.72 (dmlc#3337)

Release version 0.71 (dmlc#3200)

Update PyPI maintainer; use VERSION for binary wheels (dmlc#2992)

Tag version 0.6 (dmlc#1422)

Update CHANGES.md

Update README.md

Update README.md

Merge branch 'master' of ssh://github.com/tqchen/xgboost

fix sometimes python cachelist problem