Refactor the Extension class

Splits the class in Generic and Licesed, almost the same files we use on the AllediaInstaller, the unique difference is the namespace.
Rename the Plugin class to AbstractPlugin again

Abstract can't be a path, since it is a reserved word
Fix typo

We had a typo calling the Alledia\Framework\Joomla\Extension\Licensed class
Add a legacy class for Extension

Adds backwards compatibility with older versions of extensions that still are using this class
Merge pull request #2 from OSTraining/development

Splits the Extension class into multiple classes