Support for refreshing entries

[thomastanck]

Resolves #56.

Summary of changes:

• Rename RefreshCommand to RefreshFeedCommand to disambiguate with RefreshEntryCommand.
• Implement RefreshEntryCommand

Part of epics:

• 28 Offline support

[coveralls]

Pull Request Test Coverage Report for Build 682

• 0 of 32 (0.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.9%) to 83.773%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/logic/parser/EntryBookFeedsParser.java	0	1	0.0%
src/main/java/seedu/address/logic/parser/EntryBookListParser.java	0	1	0.0%
src/main/java/seedu/address/logic/commands/RefreshFeedCommand.java	0	2	0.0%
src/main/java/seedu/address/logic/parser/RefreshEntryCommandParser.java	0	3	0.0%
src/main/java/seedu/address/model/ModelManager.java	0	4	0.0%
src/main/java/seedu/address/logic/parser/RefreshFeedCommandParser.java	0	6	0.0%
src/main/java/seedu/address/logic/commands/RefreshEntryCommand.java	0	15	0.0%
<!--	Total:	0	32	0.0%	-->

Totals
Change from base Build 681:	-0.9%
Covered Lines:	2096
Relevant Lines:	2502

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💛 - Coveralls

[epicfailname]

Possible extension in future could be that a refresh command, without any index given, refreshes the selected entry (and prompts for index if there is no selected entry)