Z3c-recipe-compattest

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- Simplified building the list of packages even more: we now just take a list of
packages, period.

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- Restructured the way we construct our list of packages to test:
We no longer filter a list we retrieved from SVN with includes/excludes,
but use an explicit list that can be populated from a buildout section,
e. g. [versions]. Thus, we can now easily test against a KGS.
- Always enable all extras of packages under test.

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- Fix duplicate `url` parameter in setup.py that confused Python 2.4 but
got accepted by Python 2.5.

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- Ignore missing package releases for packages listed in Subversion (as
long as we don't try to run from Subversion).

- Allow parallel execution of the individual test runners by stating
'max_jobs=X' in the recipe's options.

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- Adding the exclude parameter in buildout causes the default exclude
list to be merged with the option in buildout.cfg.

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- Implemented use_svn option to use SVN trunk checkouts instead of released
versions.