Create assert_that function and use it from TestCase.assertThat.

This will allow matchers to be used outside of the TestCase context.  This is useful for people who can't buy in to testtools.TestCase fully (for whatever reason), or who like using function/generator tests (as supported by nose).

This isn't a full patch (and so shouldn't be merged as-is), but I wanted to get feedback on whether this approach would be acceptable before doing anything else.

Assuming it is acceptable, thoughts on how best to test it would be appreciated. My initial instinct was to modify all of the tests exercising the core behaviour I've extracted to point at `assert_that` and write some new tests to ensure that `assertThat` consumes the `assert_that` API correctly.

However, as we will be exposing essentially the same API in two places (`assert_that` and `self.assertThat`), I wonder if we should run the core-behaviour tests against both places by parameterising them somehow, so we're fully testing our public API. Thoughts appreciated.

This will fix https://bugs.launchpad.net/testtools/+bug/1243834 (by providing the global function mentioned in the title).