Fix some dependency issues in SBT build (same will be needed for Maven):

- Exclude a version of ASM 3.x that comes from HBase
- Don't use a special ASF repo for HBase
- Update SLF4J version
- Add sbt-dependency-graph plugin so we can easily find dependency trees
Merge branch 'master' of github.com:mesos/spark
Add Python timing instrumentation
Prefork Python worker processes
Add tests and fixes for Python daemon shutdown
Rename PythonWorker to PythonWorkerFactory
PySpark daemon: fix deadlock, improve error handling
use parens when calling method with side-effects
Fix reporting of PySpark exceptions