Allows ui factories to query users for an integer.

* bzrlib/tests/test_ui.py:
(TestTextUIFactory.test_text_ui_get_integer,
CannedInputUIFactoryTests.test_canned_input_get_input): Tests.

* bzrlib/ui/__init__.py:
(UIFactory.get_integer): New definition.
(CannedInputUIFactory.get_integer): Implementation.

* bzrlib/ui/text.py:
(TextUIFactory.get_integer): Implementation.