Revamped the BoxManager ctor; housekeeping

--The BoxManager ctor now takes an IRequestAuthenticator implementation
(v2: OAuth2RequestAuthenticator; v2:LegacyRequestAuthenticator).
Removed a problematic v1-compatible ctor.
-- Removed some v2 token refresh code from BoxManager.  It was only used
in the Program.cs sample and required several unecessary, secret pieces
information (client id, client secret, and refresh token.)