Adds unwind processor.

[pwalsh]

TODO:

• don't do some stuff when from_key == to_key['name']
• support other options on the to_key field spec? Current just name and type
• docs in PROCESSORS.md

[coveralls]

Pull Request Test Coverage Report for Build 561

• 28 of 29 (96.55%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.081%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dataflows/processors/unwind.py	27	28	96.43%

Totals
Change from base Build 559:	0.2%
Covered Lines:	2207
Relevant Lines:	2594

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💛 - Coveralls

[akariv]

Left a few comments - also don't forget to update PROCESSORS.md

[akariv]

Before yielding here, you can process the datapackage itself and remove the from_key field if source_delete and from_key != to_key.

[akariv]

I think we should also add here the 'to_key' field (with its type and other options)
(or - if from_key == to_key - update the field's type accordingly)