Bringing zombie back to life and making it compatible with 2.0.0 version

1. removing strict dependency to 0.12.15 version of Zombie (now should work with 2.0.x+)
2. using dev tests from Mink
3. using `json_encode` to escape PHP variables, used in JavaScript
4. fixed cookie setting code
5. fixed element location code (argument order)
6. special processing, when no element was found by xpath
7. removed code duplication in `click` method (for `click` event firing)
8. commented-out `isVisible` method, since there is no 100% safe way to detect this through JavaScript anyway
9. wait timeout of `wait` method was set to incorrect browser property
10. added method for getting last error (for debugging)
11. ignoring `testWait` test, because of how `wait` waits until `setTimeout` runs
12. Making `wait` method return check result (fixes #39).
13. Updating docs to indicate compatibility with 2.0.0 version
14. Adding support for window switching, since 2.0.0 version supports it (improved window tests, that pass in https://github.com/Behat/Mink/pull/400)
15. Removing non-working methods and explicitly skipping non-supported tests