Zc.relationship

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- adjust to BTrees changes in ZODB 3.8 (thanks Juergen Kartnaller)

- converted buildout to rely exclusively on eggs

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- Incorporated tests and bug fixes to relationship containers from
Markus Kemmerling:

* ManyToOneRelationship instantiation was broken

* The `findRelationships` method misbehaved if both, `source` and `target`,
are not None, but `bool(target)` evaluated to False.

* ISourceRelationship and ITargetRelationship had errors.

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- Incorporated test and bug fix from Gabriel Shaar::

if the target parameter is a container with no objects, then
`shared.AbstractContainer.isLinked` resolves to False in a bool context and
tokenization fails. `target and tokenize({'target': target})` returns the
target instead of the result of the tokenize function.

- Made README.rst tests pass on hopefully wider set of machines (this was a
test improvement; the relationship index did not have the fragility).
Reported by Gabriel Shaar.

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Initial release