Support for Set Metadata

[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 1878

• 41 of 45 (91.11%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 62.512%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxFile.java	20	22	90.91%
src/main/java/com/box/sdk/BoxFolder.java	20	22	90.91%

Totals
Change from base Build 1875:	0.4%
Covered Lines:	5041
Relevant Lines:	8064

---

💛 - Coveralls

[mattwiller]

Left some minor documentation comments, but the implementation looks good to me!