Update src/libcmdline/CommandLine.cs

Update exception message to describe in more detail what the exception is.
Merge pull request #10 from hugodahl/master

Display all missing but required values.
Version changed to 1.9.3.21 beta after hugodahl pull request.
Version updated in documents.
Issue about parsing negative values fixed.
NuGet package definition file updated.
Fixes numbers overflow (issue gh user jandler).
Merge pull request #11 from jafin/patch-1

Update exception message for shortName attribute option to describe in more detail what the exception is.
Consolidation of pull request #11 by github user jafin.
Line endings converted to Unix format.
Added HeadingInfo.Default and CopyrightInfo.Default.

HeadingInfo.Default exposes the default heading generated by
HelpText.AutoBuild.
Sensible defaults have also been added, removing the need to explicitly
add assembly attributes (though they still take precedence).

CopyrightInfo.Default exposes the default info generated by
HelpText.AutoBuild.
Similar to HeadingInfo.Default.

Also fixes bug #22.
ReSharper refactoring and fix of issue #22.
Merge with master.
Added missing XML doc comments; assembly signed.
nuget .nuspec updated and package pushed to repository.
VS2012 saved it with wrong line endings; reconverted to *nix.
Test project refactored using Should.Fluent library.
Extended use of Should.Fluent to whole test project.
Updated .nuspec and relative readme file.
Added ReSharper code style file.