Add the users request/response builders and managers, as well as the user's model
Expose usersManager in BoxSDK and BoxCocoaSDK

Include the new .h files in the framework header, create users manager
when creating sharedSDK singleton.

This commit also includes the new files and tests for the BoxCocoaSDK.
Add comment_count to file object.

https://github.com/box/box-ios-sdk-v2/issues/28
Change a code-comment for BoxFile.commentCount.
Merge pull request #43 from RicoYao/add_comment_count_to_file_object

Add comment_count to file object.
Update READMEs to reflect addition of users endpoint
Merge branch 'master' of https://github.com/box/box-ios-sdk-v2

Conflicts:
	BoxSDK.xcodeproj/project.pbxproj
	BoxSDK/BoxSDK.h
Fix completion callback when blocks are nil