Make add command save webpages to disk

[thomastanck]

Resolves #22.

Next part after this is to allow loading of local webpages into the WebView.

Summary of changes

• Added ArticleStorage and related classes to manage saving articles to disk. No articles are kept in memory. Instead, a Path can be obtained in order to load the article required into a WebView.
• Modified UserPrefs and Model accordingly to include the right paths and methods for ArticleStorage.
• Refactored URLs used for Network tests into TypicalEntries.
• Added unit and integration tests.
• Connected article saving to the add command.

[coveralls]

Pull Request Test Coverage Report for Build 168

• 45 of 50 (90.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 93.893%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/storage/DataDirectoryArticleStorage.java	17	22	77.27%

Totals
Change from base Build 153:	-0.1%
Covered Lines:	1522
Relevant Lines:	1621

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💛 - Coveralls

[rlrh]

Very nice work!

Just a question: why do we not store the downloaded files with a .htm or .html extension?

[thomastanck]

@rlrh thanks for the review!

I left it out for the moment as that wasn't necessary to meet the requirements and also because I would prefer if the extension on the url matched the extension saved (may not always be .html). It also complicates things when there is no extension.