Extract duplicate classes in reef-examples

This pull request addresses https://issues.apache.org/jira/browse/REEF-19

* Currently there have been duplicate classes in some of the reef examples - ShellTask and Command.
* Further, the ShellTask in RetainedEval is outdated, even it does the exactly same thing with the one in HelloHttp.
* The classes are moved into 'o.a.r.examples.library'.
* RetainedEval and HelloHttp are tested after modification.