Refactor Basic Authentication package

This change refactors the basic authentication implementation to better follow
Go coding standards. Many types are no longer exported. The parser is now a
separate function from the authentication code. The standard functions
(*http.Request).BasicAuth/SetBasicAuth are now used where appropriate.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/config.yml

@@ -26,10 +26,6 @@ storage:
     maintenance:
         uploadpurging:
             enabled: false
-auth:
-    basic:
-       realm: test-realm
-       path: /tmp/registry-dev/.htpasswd
 http:
     addr: :5000
     secret: asecretforlocaldevelopment