Dripper

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- Avoid deepcopy to improve speed

- https://github.com/hirokiky/dripper/pull/6
- Thanks afiram

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- ``None`` is default value of ``ValueDripper``

- Before this change ``ValueDripper`` without ``default`` keyword argument
will raise ``DrippingError``
- In order to this behavior ``DictDripper`` will return empty dict
when inner value dripper could not dig out values
- Thanks for bungoume to suggest this behavior

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- Officially supported Python 3.5

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- ValueDripper now accepts ``default`` argument.

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- Fixed to accept string or integer directly as source_root.

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- Improved error handling.
- Added MixDripper.