Two more propositions on how to deal with 3rd party extension (no direct editions and what to do in case of a rewrite conflict)

Two more propositions on how to deal with 3rd party extension: 
* no direct editions (because such changes may be lost when the extension is updated) 
* what to do in case of a rewrite conflict (when two or more modules rewrite the same blocks/models, only the one parsed last will be working correctly - in this case it's necessary to create a local module that <code>depends</code> on every conflicting module (so it's loaded last) and rewrites the same things (so a developer can ensure that it includes all the pieces of code needed for conflicting modules to work correctly). A good practice is to <code>extend</code> the final models/blocks using one of conflicting classes, so its code doesn't have to be repeated (this approach won't work if conflicting modules make changes to the same class methods). This piece of advice is not included in the commit, because it would make the text a bit too long - I think we don't want to transform the magentotherightway.com into a 300-pages rule book just now. ;-)