at() methods should return the page object on match

Originally created by Luke Daley.

This would make it easier for people wanting to use more types:

```
class SomeGebSpec extends GebSpec {

    def "login"() {
        when:
        go "http://somewhere.com"

        then:
        def loginPage = at(LoginPage)
        loginPage.username.value "fred"
    }
}
```
