Added SPARK_WORKER_INSTANCES and SPARK_WORKER_CORES support
Added SPARK_MASTER_OPTS configuration
Added spark_master_opts to README
Avoid workers with no cores when over-subscribing workers
Merge pull request #46 from douglaz/configurable_workers

Configurable workers
HOTFIX: Backwards-compatiblity fix for #46
Merge pull request #47 from pwendell/hotfix

Hotfix
update shark env
Merge pull request #48 from haoyuan/v3

update shark env