(*) Added Command as base class for TaskCommand.
(+) Added base class for generic command classes. Contains protected static services for using in derived classes.
(-) Removed bad check in Command class in CanExecuteChanged {add/remove} methods causes prevention of subscribing to the event in case when can-execute Func<bool> and Func<TCanExecuteParameter,bool> functions are nulls, but there could be other conditions to check can command be executed or not (e.g. AuthenticationProvider check and any other conditions in overridden Command<>.CanExecute method).
(*) TaskCommand now supports WIN80 platform.
Added comment to a public TaskCommand method.
(*) TaskCommand now supports WP8 platform.
Merge pull request #7 from inTagger/develop

Refactored TaskCommand/Command. Added WIN8/WP8 support.