Support 'exists' test in factory

[tonioo]

Add support for 'exists' tests when using the factory.

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• Bitbucket: https://bitbucket.org/tonioo/sievelib/issue/25
• Originally Reported By: Antoine Nguyen
• Originally Created At: 2013-12-16T15:05:49.194

[tonioo]

Adding support for negative tests and 'exists' tests.

fixes #24
fixes #25

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Original Comment By: Antoine Nguyen