[WIP] Mongo db backend updated#17

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sibiryakov:MongoDb-backend

Commits on Feb 9, 2015

Extracting methods responsible for 'page' and request creation for memory and sqlalchemy-based backends.
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Commits on Mar 2, 2015

Merge pull request #10 from sibiryakov/sqlalchemy-decomposition
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Commits on Mar 16, 2015

MongoDB backend.
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