Add tags support

Add support for "tags" field of files and folders, and "my_tags" field
of users.
Merge pull request #71 from mmorearty/tags

Add tags support
modify unit tests for the tag support
Merge pull request #75 from box/addutest

modify unit tests for the tag support
update javadoc
Merge pull request #76 from box/jd

update javadoc
Updated name
Merge branch 'master' of github.com:box/box-java-sdk-v2 into login_token