Added tests for pages via Mink

This tests the whole library and pages without mocking via Mink and the Selenium Driver.
Just now only basic but we can improve them and add more later.
I suppose `$page->checkbox` is `WebElement`, which extends `NodeElement`. If so, then you can use dedicated `check` and `uncheck` methods of a Mink.

FYI clicking on checkbox will check/uncheck it only in Selenium2 driver.