cache processor, nested flows, load from datapackage

[OriHoch]

this PR contains the following changes (all related to the requirements of the cache processor):

• cache processor - simple cache processor which caches results from a series of steps to a provided cache path
• nested flows - support using a Flow object as a step in another Flow
• load processor - added support for loading resources from a datapackage

see updated docs for details and example usage

[coveralls]

Pull Request Test Coverage Report for Build 37

• 44 of 44 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 73.845%

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Totals
Change from base Build 32:	0.7%
Covered Lines:	847
Relevant Lines:	1147

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💛 - Coveralls

[OriHoch]

@akariv updated