Track task value serialisation time in TaskMetrics.
UI to display serialisation time of a stage.
Incorporate pwendell's code review suggestions.
Merge branch 'master' of git://github.com/apache/incubator-spark
Missed a spot; had an objectSer here too.
objectSer -> valueSer in a test.
Add a serialisation time column to the StagePage.
Merge pull request #272 from tmyklebu/master

Track and report task result serialisation time.

 - DirectTaskResult now has a ByteBuffer valueBytes instead of a T value.
 - DirectTaskResult now has a member function T value() that deserialises valueBytes.
 - Executor serialises value into a ByteBuffer and passes it to DTR's ctor.
 - Executor tracks the time taken to do so and puts it in a new field in TaskMetrics.
 - StagePage now reports serialisation time from TaskMetrics along with the other things it reported.