Build authorization header, check for missing oauth_token and oauth_consumer_key

Build authorization header, check for missing oauth_token and oauth_consumer_key

Add method to Horde_Oauth_Request that builds an OAuth Authorization header
(http://oauth.net/core/1.0#auth_header)

Allow H_O_Request to obtain the *required* oauth_consumer_key and the oauth_token
values from the Consumer and Token objects respectively so that client code doesn't
have to awkardly pass them in both the Horde_Oauth_Consumer const'r and in the
Horde_Oauth_Request const'r

Add method to build an OAuth Authorization Header