Add support for sorting folder items

[carycheng]

No description provided.

[boxcla]

Hi @carycheng, thanks for the pull request. Before we can merge it, we need you to sign our Contributor License Agreement. You can do so electronically here: http://opensource.box.com/cla

Once you have signed, just add a comment to this pull request saying, "CLA signed". Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 1866

• 11 of 12 (91.67%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.006%) to 62.09%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxFolder.java	11	12	91.67%

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/EventStream.java	1	87.5%
src/main/java/com/box/sdk/RealtimeServerConnection.java	3	80.0%

Totals
Change from base Build 1863:	-0.006%
Covered Lines:	4979
Relevant Lines:	8019

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1869

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.007%) to 62.103%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/EventStream.java	1	87.5%
src/main/java/com/box/sdk/RealtimeServerConnection.java	3	80.0%

Totals
Change from base Build 1863:	0.007%
Covered Lines:	4980
Relevant Lines:	8019

---

💛 - Coveralls