[REEF-79]: Add a script that enables running REEF-Tests on YARN.

Changes:
* Added the script (thanks to @dafrista, @markusweimer, @bgchun)
* Added test-jar to include test code in test-jar-with-dependencies, which we will run tests against



How to run reef-tests on YARN:

1. Build reef by running mvn clean install -DskipTests 
2. Build reef-tests jar by running mvn jar:test-jar from the reef-tests directory
3. Run the tests by running bin/runyarntests.sh

It would be safer to derive this classpath from the `yarn classpath` command which returns the classpath YARN applications should use.