Ftw.testing

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- Add savepoint simulation to transaction interceptor. [jone]

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- Do not require "Plone" egg. [jone]

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- Add transaction interceptor. [jone]

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- Support Plone 5.1 [mathias.leimgruber, jone]
- Remove splinter browser. Use ftw.testbrowser instead. [mathias.leimgruber, jone]
- Drop Plone 4.1 support. [jone]

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- Freezer: reimplement "now" patching with forbiddenfruit.
This fixes problems with pickling and comparison of frozen datetime objects.
[jone]

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- Freezer: disable freezing while committing to database for preventing pickling errors.
[jone]

- Freezer bugfix: replace datetime instances when leaving freeze context manager.
[jone]