[WIP] Added Cassandra backend#225

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voith wants to merge 14 commits intoscrapinghub:masterfrom

voith:PR-128-cassandra-backend

Commits on Oct 29, 2016

Added Cassandra Backend

MichaelVIU
authored and
voith
committed

Commits on Oct 30, 2016

refactored some sqlalchemy code to reuse in cassandra backend
voith
committed

Commits on Oct 31, 2016

added unit tests for cassandra backend
voith
committed

Commits on Nov 1, 2016

added pickle field to serialize dicts in cassandra
voith
committed

Commits on Nov 2, 2016

refactored cassandra metadata and added tests for it
voith
committed

Commits on Nov 3, 2016

fix pickledict bugs in py2
voith
committed

Commits on Nov 4, 2016

refactored cassandra states and added tests for it
voith
committed

Commits on Nov 5, 2016

added cassandra queue and tests for it
voith
committed

Commits on Nov 6, 2016

Added Lifo, fifo, dfs, bfs backend and correspoding tests for it
voith
committed

Commits on Nov 7, 2016

fixed connection issue in tests
voith
committed

Commits on Nov 8, 2016

Added cassandra revisiting backend and tests for it
voith
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Commits on Nov 9, 2016

added unitests for utcnow_timestamp
voith
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Commits on Nov 10, 2016

updated cassandra docs
voith
committed

Commits on Nov 11, 2016

added tests for cassandra distributed backend
voith
committed