Add membership related features

[sohtsuka]

Closes #77.

[boxcla]

Verified that @sohtsuka has signed the CLA. Thanks for the pull request!

[gcurtis]

Good thinking documenting this. Can we just rephrase this comment (as well as the others) to say something like:

"Note: the BoxUser.Info returned by this method will only have the ID, name, and login fields populated."

[gcurtis]

I've been trying to figure out why updateMembershipSendsCorrectRequestAndParsesResponseCorrectly() is failing. When I run it by itself with:

gradle test --tests *updateMembershipSendsCorrectRequestAndParsesResponseCorrectly

it passes, but when it's run with the other BoxGroupMembership tests, it fails. I'm wondering if this is an issue with WireMock.

If you can't figure what's wrong, then I'm okay with just deleting that test since you also wrote an integration test that covers updating info.

[sohtsuka]

Thanks for your comments.
As I mentioned on the pull request #100, I have been getting similar errors since before this commit. All the tests pass if I comment out the following line in BoxAPIRequest.java (line 345):

connection.setFixedLengthStreamingMode((int) this.bodyLength);

but I can't figure out why the errors pop up. I will just delete updateMembershipSendsCorrectRequestAndParsesResponseCorrectly().

[gcurtis]

Thanks for the contribution!