Add User class for retrieving Highrise accounts.

Many Highrise objects reference 'author_id' properties that relate to
the id of the user account responsible for the action - e.g. adding a
note to a person or Company. This new class, User, represents these
user accounts, and allows you to retrieve the real name / email of
the user, so that you can say 'note added by Bob', rather than
'note added by user 12345'.