Support for yard-doctest

[yard-doctest](https://github.com/p0deje/yard-doctest) has been my weekend project, but I believe watir can benefit from it because it ensures that all codebase examples are actually valid. This is of course not a replacement for specs, just an addition to docs we have.

This is how the run looks like:

```bash
$ rake yard:doctest
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/browser.rb' near line 254
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/browser.rb' near line 271
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/user_editable.rb' near line 10
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/user_editable.rb' near line 10
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/user_editable.rb' near line 25
[warn]: @param tag has unknown parameter name: *args
    in file `lib/watir-webdriver/user_editable.rb' near line 25
Run options: --seed 64595

# Running:

..........................................

Finished in 208.747670s, 0.2012 runs/s, 0.0623 assertions/s.

42 runs, 13 assertions, 0 failures, 0 errors, 0 skips
```

@jarib @jarmo What do you think about this?
`#png` and `#base64` assertions are removed since I see no obvious way to figure out the expected result.