[SPARK-1812] Scala 2.11 support.

Changes since previous PRs
1) removed all sbt version revert or changes to jenkins scripts.
2) Removed SCALA_VERSION -> SPARK_SCALA_VERSION changes from windows scripts.
3) now we set spark.executor.extraClasspath during build for both maven and sbt.

and few more ...

I can prepare separate PR for new build  instruction.

Anyone looking at this can build for scala 2.11 by using the build profile -Pscala-2.11.

if you build using maven don't forget dev/change_X_to_Y.sh scripts before you build.
This is so much nicer than it is in maven :P It would be good to add a comment here explaining what this is for.