Introduce a way to use custom time source for time validation (issue #364)

[thekondor]

decode() is extended with an option to use custom UTC current time value to solve #364. Tests are provided.

I decided to use an approach to specify a custom UTC current time's value through using options argument; since it looks more organic for the current implementation. Another possible approach could be is passing utcnow() function through a setter function like register_algorithm().

[coveralls]

Pull Request Test Coverage Report for Build 17

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 76.87%

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Totals
Change from base Build 15:	0.2%
Covered Lines:	555
Relevant Lines:	722

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 17

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 76.87%

---

Totals
Change from base Build 15:	0.2%
Covered Lines:	555
Relevant Lines:	722

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💛 - Coveralls