Mongopersist

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- Bug: Do not fail if we cannot delete the parent and name attributes.

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- Feature: Switch to ``pymongo.MongoClient``, set default write concern values,
allow override of write concern values.

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- Tests: Added, cleaned tests

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- Bug: Due to ``UserDict`` implementing ``dict`` comparison semantics, any
empty ``MongoContainer`` would equate to another empty one. This behavior
would cause object changes to not be properly recognzed by the mongo data
manager. The implemented solution is to implement default object comparison
behavior for mongo containers.

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- Feature: Update to latest package versions, specifically pymongo 2.4.x. In
this release, ``pymongo`` does not reexport ``objectid`` and ``dbref``.

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- Bug: avoid caching MongoDataManager instances in mongo container to avoid
multiple MongoDataManagers in the single transaction in multithreaded
environment. Cache IMongoDataManagerProvider instead.