Ftw-testing

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- Implement Generic Setup uninstall base test.
[jone]

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- Fix ``isinstance`` calls of freezed time in ``freeze`` context manager.
[jone]

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- Implement ``freeze`` context manager for freezing the time.
[jone]

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- AT form page object: add schemata helper methods for testing visible
schematas and fields.
[jone]

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- Add custom mailhost class, remembering the sender and recipient
of each email separately.
[deif]

- Deprecate javascript because Selenium with PhantomJS is too unstable.
Removes tests and documentation, the javascript decorator still works
for now but needs to be imported from ftw.testing.browser.
[jone]

- Page objects: add a Plone.visit(obj) function.
[jone]

- Fix a rare bug where the MockMailHost message list has been replaced by
another instance.
[jone, deif]

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- Move ``Mailing`` helper class to its own module ``mailing``.
[deif]