W.I.P Extension meta data

I'm trying to add the feature without refactoring too much code, but a more formalized solution, with the proper interfaces and secondary classes, will come later.

Meeds discussion:

- core extensions have no package.xml. What would jesus do?
- previously downloaded extensions (before ExtensionInstaller latest update) are failing because their package.xml was not preserved. Commit 67f20d8 takes care of preserving xml;



PS: This is a work in progress pull request