ENH: Add property as a supported type.

[ehebert]

Enables definitions such as:

class FooInterface(Interface):

    @property
    def bar(self):
        pass

class Foo(implements(FooInterface)):

    @property
    def bar(self):
        return 'baz'

If the implementation is missing the property decorator, such as:

class FooInterface(Interface):

    @property
    def bar(self):
        pass

class Foo(implements(FooInterface)):

    def bar(self):
        return 'baz'

Then an error is raised showing that the method has the wrong type:

The following methods of FooInterface were implemented with incorrect types:
  - bar: 'function' is not a subtype of expected type 'property'