(Asynchronously) Download undownloaded articles on select

[thomastanck]

Resolves part of #169, #156.

Summary of changes:

• Add async interface to Network
• On selection, check if entry is un-downloaded, and download it if so.

[coveralls]

Pull Request Test Coverage Report for Build 769

• 27 of 28 (96.43%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 83.7%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/model/ModelManager.java	10	11	90.91%
<!--	Total:	27	28	96.43%	-->

Totals
Change from base Build 766:	0.2%
Covered Lines:	2090
Relevant Lines:	2497

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💛 - Coveralls