[jvm-packages] Integration with Spark Dataframe/Dataset (#1559)

* bump up to scala 2.11

* framework of data frame integration

* test consistency between RDD and DataFrame

* order preservation

* test order preservation

* example code and fix makefile

* improve type checking

* improve APIs

* user docs

* work around travis CI's limitation on log length

* adjust test structure

* integrate with Spark -1 .x

* spark 2.x integration

* remove spark 1.x implementation but provide instructions on how to downgrade

.gitignore

@@ -79,3 +79,5 @@ tags
 *.class
 target
 *.swp
+
+.DS_Store