Feature/add group operation

[Kalzem]

#14

This PR introduces the class GroupOperation which can include a few ConcurrentOperations that will run in parallel. Each ConcurrentOperation is responsible for it failures and retries. Doing so doesn't change Queuer's logic at all.

Allows for such flows: 1 -> [2A || 2B || 2C] -> 3

[Kalzem]

It's strange that Travis failed because of gem ?

[coveralls]

Pull Request Test Coverage Report for Build 447

• 13 of 14 (92.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 96.957%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Sources/Queuer/GroupOperation.swift	13	14	92.86%

Totals
Change from base Build 430:	-0.3%
Covered Lines:	223
Relevant Lines:	230

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💛 - Coveralls

[FabrizioBrancati]

I restarted the job, but on Linux, the tests don't want to pass

[FabrizioBrancati]

All tests succeded 🎉