[Feature] Verifying if a page is open

Fixes #29
Replaces #15

* The open method now calls `verifyUrl()` in addition to other vierfy methods (`verifyResponse()` and `verifyPage()`). All of verification methods are extracted to a single `verify()` call, so it's easier to override them.
* The new `isOpen()` method calls the same verification methods as `open()` and returns boolean.
* The new `getUrl()` method returns the page url.

See acceptance tests for more details.