code & unit test for spark context hang detection logic
code update based on code reviews
Merge branch 'master' of https://github.com/spark-jobserver/spark-jobserver into sc-hang
code changed based on code review requirements
Merge remote-tracking branch 'open/master' into sc-hang

Conflicts:
	project/Dependencies.scala
Merge pull request #5 from spark-jobserver/sc-hang

Code & unit test for detecting hang Spark workers