* rank_metric: add AUC-PR
Implementation of the AUC-PR calculation for weighted data, proposed by Keilwagen, Grosse and Grau (https://doi.org/10.1371/journal.pone.0092209)
* rank_metric: fix lint warnings
* Implement tests for AUC-PR and fix implementation
* add aucpr to documentation for other languages
* fix rebase conflict
* [core] additional gblinear improvements
* [R] callback for gblinear coefficients history
* force eta=1 for gblinear python tests
* add top_k to GreedyFeatureSelector
* set eta=1 in shotgun test
* [core] fix SparsePage processing in gblinear; col-wise multithreading in greedy updater
* set sorted flag within TryInitColData
* gblinear tests: use scale, add external memory test
* fix multiclass for greedy updater
* fix whitespace
* fix typo
* [R] make sure things work for a single split model; fixes#2191
* [R] add option use_int_id to xgb.model.dt.tree
* [R] add example of exporting tree plot to a file
* [R] set save_period = NULL as default in xgboost() to be the same as in xgb.train; fixes#2182
* [R] it's a good practice after CRAN releases to bump up package version in dev
* [R] allow xgb.DMatrix construction from integer dense matrices
* [R] xgb.DMatrix: silent parameter; improve documentation
* [R] xgb.model.dt.tree code style changes
* [R] update NEWS with parameter changes
* [R] code safety & style; handle non-strict matrix and inherited classes of input and model; fixes#2242
* [R] change to x.y.z.p R-package versioning scheme and set version to 0.6.4.3
* [R] add an R package versioning section to the contributors guide
* [R] R-package/README.md: clean up the redundant old installation instructions, link the contributors guide
* [R] xgb.save must work when handle in nil but raw exists
* [R] print.xgb.Booster should still print other info when handle is nil
* [R] rename internal function xgb.Booster to xgb.Booster.handle to make its intent clear
* [R] rename xgb.Booster.check to xgb.Booster.complete and make it visible; more docs
* [R] storing evaluation_log should depend only on watchlist, not on verbose
* [R] reduce the excessive chattiness of unit tests
* [R] only disable some tests in windows when it's not 64-bit
* [R] clean-up xgb.DMatrix
* [R] test xgb.DMatrix loading from libsvm text file
* [R] store feature_names in xgb.Booster, use them from utility functions
* [R] remove non-functional co-occurence computation from xgb.importance
* [R] verbose=0 is enough without a callback
* [R] added forgotten xgb.Booster.complete.Rd; cran check fixes
* [R] update installation instructions
* [R-package] JSON tree dump interface
* [R-package] precision bugfix in xgb.attributes
* [R-package] bugfix for cb.early.stop called from xgb.cv
* [R-package] a bit more clarity on labels checking in xgb.cv
* [R-package] test JSON dump for gblinear as well
* whitespace lint
