Gocept-selenium

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- Improved documentation, in particular with respect to the changes by
integrating webdriver.

- If an ``AssertionError`` occures in a test using webdriver, a screenshot
is taken automatically and the path is presented to the user. (12247)

- Made a test for ``assertScreenshot`` pass on systems with a different
browser default font.

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- Stabilize webdriver/selenese API functions `waitForPageToLoad()` and
`isTextPresent` to not raise errors when the elements vanish in between.

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- Extract StaticFilesLayer to gocept.httpserverlayer.

- Added `assertScreenshot` to visually compare rendered elements with a
master screenshot.

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- Add layer that uses Webdriver as the Selenium backend instead of the old
Remote Control.

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- Fix: Initialise the WSGI layer in the correct order to actually allow the
configured WSGI app to be remembered.

- Fix: updated some imports after the extraction of gocept.httpserverlayer.

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- Update StaticFilesLayer to the new httpserverlayer API.