adds shared link downscoping and coverage

[boxdave]

• Adds string sharedLink arg thats a Box shared link to BoxAPIConnection.getLowerScopedToken. Returns a downscoped token for the resource associated to the shared link if the args not null.
• Adds test coverage.

[PJSimon]

@boxdave I think the comments from Cary can be resolved, right?

[coveralls]

Pull Request Test Coverage Report for Build 2257

• 24 of 42 (57.14%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 65.933%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxAPIConnection.java	24	42	57.14%

Totals
Change from base Build 2253:	-0.05%
Covered Lines:	5630
Relevant Lines:	8539

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💛 - Coveralls