Explicit relative imports

This change allows to declare relative imports to the module namespace.

    namespace Foo.Bar

    import .Baz                    # Foo.Bar.Baz
    import ..Qux                   # Foo.Qux
    from ...Boo.Lang import List   # Boo.Lang.List

The reasoning for this feature is to bring Boo a bit more in line with interpreted languages. If a project is modeled on a directory structure mapped 1:1 to namespaces, it's useful to easily define relative imports and tell them apart from absolute ones.

This syntax will become even more useful if at some point it's possible to infer the module namespace without explicitly declaring it.