Gocept.selenium

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- Webdriver: No longer screenshotting while waiting for the condition to
become true when using a ``waitFor*`` method.

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- Remove seleniumrc variable from Layer on teardown for symmetry.

- Fix `isVisible` of WebDriver, so it also returns False if a parent element
is hidden.

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- Close temporary files when making screenshots. This fixes some occurrences
of "Too many open files".

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- Make timeout configurable via environment variable
``GOCEPT_SELENIUM_TIMEOUT`` (10497).

- Apply ``setTimeout`` to the ``open()`` timeout, too (10750).

- Add environment variable ``GOCEPT_SELENIUM_JUNIT_ATTACH`` to support the
"JUnit Attachments Plugin" for Jenkins.

internal:

- Move instantiating Selenese object from testSetUp to layer setUp. This
*should* not change the behaviour for clients (we take care to reset the
configured timeout in testSetUp as before), but take care.

- Fix URL to GROK toolkit versions.

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- Marking 2.0 stable, yay.

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- Save screenshots of assertion failures with mode 644 (world-readable),
which is useful for build servers.