If Review Board is to support OAuth2 applications, we will need a way to

indicate to API consumers the currently logged in user. Currently, there

is no way to determine the currently logged in user. Now the root

resource will serialize a link to the user resource for the

currently-logged in user so that API clients can retrieve user details

to use for identifying users and retrieving user details. This will

enable services to "Log In with Review Board" and populate their

database with user information, such as RB username, user full name,

e-mail, etc.

Manually tested the API.

Ran unit tests.