Make the decorators attribute on View a tuple by default

This should not be mutable by default, since it too easily allows users to modify the default `decorators` on any `View` class, for instance by doing something like

    MyView.decorators.append(my_decorator)

Since `list`s are mutable, unless they have explicitly declared `decorators` in the `MyView` class definition as well, this will actually append to `View`s `decorators` list, thus forcing it on all `View`s in their project that don't explicitly define `decorators`. This happened to me when writing some unit-tests for a decorator, and took a while to pin down exactly what was going on.

I haven't updated the documentation to show users supplying their lists of decorators in tuple syntax, since it looks a bit weird given `methods` right above it is a list. Let me know if you do want me to update this as well, though, or add a warning note about this.