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, or you need to specify short name in the class? You would get exception " Can't register non-existent short name of named parameter: org.apache.reef.tang.formats.NamedParameters$StringNoShortNameNoDefault” if you use StringNoShortNameNoDefault class here.