Add weights support to xgbTree model

[schistyakov]

According to xgboost documentation we can set items weights through the xgb.DMatrix. The default value for weights in caret::train is NULL, so the default behavior will stay the same

[coveralls]

Coverage remained the same at 16.533% when pulling cd7abc2 on schistyakov:xgbTree-weights into 1553656 on topepo:master.

[topepo]

Can you add some new test cases to the regression tests and also add a note in the news file?

[schistyakov]

OK, will do it

[coveralls]

Coverage increased (+2.01%) to 18.544% when pulling 71ff4f4 on schistyakov:xgbTree-weights into 1553656 on topepo:master.

[coveralls]

Coverage increased (+2.01%) to 18.544% when pulling 71ff4f4 on schistyakov:xgbTree-weights into 1553656 on topepo:master.

[topepo]

Thanks