Compatibility with zope.interface < 3.6.0.

`implementer` decorator does not work on classes before zope.interface
3.6.0. Ubuntu Lucid ships with zope.interface 3.5.3. This commit
replaces the use of `implementer` decorators by the old way to declare
an implementation.

Of course, maybe compatibility with Ubuntu Lucid is not important enough to change a clean decorator in some crappy function call.