REEF-34: Add convencience methods for command line parsing

This addresses [REEF-34](https://issues.apache.org/jira/browse/REEF-34) and adds the two methods mentioned.
Should StringNoShortNameNoDefault be replaced with StringShortNameDefault? You would get exception "Can't register non-existent short name of named parameter:…” if you use StringNoShortNameNoDefault class here.