Added event OptionParsed on OptionSet

Added a new event "OptionParsed" on OptionSet that fires whenever one of the options it contains has finished parsing, containing the parsed OptionContext.
Style: method names are PascalCased.

[FxDG](http://msdn.microsoft.com/en-us/library/ms229011(v=vs.110).aspx): This should be a `protected` (+`internal`) `virtual` method:

> √ **DO** use a protected virtual method to raise each event. This is only applicable to nonstatic events on unsealed classes, not to structs, sealed classes, or static events.

    internal protected virtual void OnOptionParsed (OptionParsedEventArgs e)
    {
        var p = OptionParsed;
        if (p != null)
            p (this, e);
    }