Merge pull request #384 from squito/executor_spark_home

executor can use a different sparkHome from Worker 
Only bind to IPv4 addresses when trying to auto-detect external IP
Merge branch 'branch-0.6' of github.com:mesos/spark into branch-0.6
Clarify the documentation on env variables for standalone mode
Add more Akka settings to docs
be sure to set class loader of kryo instances
Merge branch 'kryo-class-loader' into imran/0.6-debug
Merge branch 'branch-0.6' into imran/0.6-debug