Merge pull request #311 from tmyklebu/master

SPARK-991: Report information gleaned from a Python stacktrace in the UI

Scala:

- Added setCallSite/clearCallSite to SparkContext and JavaSparkContext.
  These functions mutate a LocalProperty called "externalCallSite."
- Add a wrapper, getCallSite, that checks for an externalCallSite and, if
  none is found, calls the usual Utils.formatSparkCallSite.
- Change everything that calls Utils.formatSparkCallSite to call
  getCallSite instead. Except getCallSite.
- Add wrappers to setCallSite/clearCallSite wrappers to JavaSparkContext.

Python:

- Add a gruesome hack to rdd.py that inspects the traceback and guesses
  what you want to see in the UI.
- Add a RAII wrapper around said gruesome hack that calls
  setCallSite/clearCallSite as appropriate.
- Wire said RAII wrapper up around three calls into the Scala code.
  I'm not sure that I hit all the spots with the RAII wrapper. I'm also
  not sure that my gruesome hack does exactly what we want.

One could also approach this change by refactoring
runJob/submitJob/runApproximateJob to take a call site, then threading
that parameter through everything that needs to know it.

One might object to the pointless-looking wrappers in JavaSparkContext.
Unfortunately, I can't directly access the SparkContext from
Python---or, if I can, I don't know how---so I need to wrap everything
that matters in JavaSparkContext.

Conflicts:
	core/src/main/scala/org/apache/spark/api/java/JavaSparkContext.scala