harvester: add arXiv metadata harvester

[coveralls]

Pull Request Test Coverage Report for Build 240

• 14 of 44 (31.82%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 64.662%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asclepias_broker/harvester/arxiv.py	14	44	31.82%
<!--	Total:	14	44	31.82%	-->

Totals
Change from base Build 238:	-0.8%
Covered Lines:	1226
Relevant Lines:	1896

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💛 - Coveralls