Pass SBT_OPTS environment through to sbt_launcher
Revert "Merge pull request #841 from rxin/json"

This reverts commit 1fb1b0992838c8cdd57eec45793e67a0490f1a52, reversing
changes made to c69c48947d5102c81a9425cb380d861c3903685c.
Added worker state to the cluster master JSON ui.
Merge pull request #864 from rxin/json1

Revert json library change
Merge pull request #859 from ianbuss/sbt_opts

Pass SBT_OPTS environment through to sbt_launcher
Add YarnClientClusterScheduler and Backend.

With this scheduler, the user application is launched locally,
While the executor will be launched by YARN on remote nodes.

This enables spark-shell to run upon YARN.