invenio.exe.menu: Menu extension created

* Adds an extension which provides menus organised in a tree structure
* Menu items are created through decorators
* Menus can be displayed in templates

Signed-off-by: Krzysztof Lis <krzysztof.lis@cern.ch>
WebTag: example usage of MenuAlchemy
Merge pull request #16 from Demestotenes/code-structure

MenuAlchemy