Removed requirement to have short name for every option.

Implementation for issue #14.

<code>CommandLineOption</code> can now be constructed with a null/empty/whitespace short name, provided a long name is also specified. New tests added to reflect this (in <code>FluentCommandLineParserTests</code>).

<code>Setup&ltT&gt(string shortOption, string longOption)</code> throws an <code>ArgumentOutOfRangeException</code> for <code>shortOption</code> if neither is specified.

The example in <code>FluentCommandLineParserTests</code> is updated to include an option which has only a long name and no specified short name, and also with an option group to ensure that this functionality is not broken by the changes.