Gocept.runner

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- Fix faulty release 0.7.0.

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- An error message is correctly logged when the worker produces an exception
which cannot be represented in us-ascii. This used to cause an exception in
the logging module. The cause of those exceptions where quite hard to track.

- Removed dependency on zope.app.security which was neither declared nor
necessary.

- Moved project home to <https://bitbucket.org/gocept/gocept.runner/>.

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- Added transaction_per_item decorator.
- Use stdlib's doctest instead of zope.testing.

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- Use log.error/log.warning instead of log.exception. Conflict errors are
logged as warning now, because they are not really a problem.

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- Convert logged exceptions to str.

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- Declared dependencies correctly.