Expose extension point for generic OAuth providers

Further development of OAuth authentication scheme support suggested in
I86fb8fab3 is to restrict the core to expose only the OAuth extension
point and use Gerrit plugin concept for OAuth provider implementations.

When multiple OAuth providers are deployed on Gerrit site (from one or
multiple plugins) selection page is shown to select OAuth provider per
user base (as it's known for OpenID authentication scheme). The only
difference is that the user can only select between deployed providers.

OAuth logo was borrowed from:

http://en.wikipedia.org/wiki/OAuth and
http://en.wikipedia.org/wiki/File:Oauth_logo.svg
The OAuth logo, designed by Chris Messina
Creative Commons Attribution-Share Alike 3.0 Unported license

Converted as base64 using: http://www.base64-image.de
Source for OAuth protocol description: the same link as above.

Bug: issue 2677
Bug: issue 2715
Contributed-by: Luca Milanesio <luca.milanesio@gmail.com>
Change-Id: I7da0a6b3f2a99b6188bd14cf2818f673a3ddd680

gerrit-server/src/main/java/com/google/gerrit/server/config/AuthConfig.java

@@ -197,7 +197,7 @@ public class AuthConfig {
       case LDAP_BIND:
       case CLIENT_SSL_CERT_LDAP:
       case CUSTOM_EXTENSION:
-        // Its safe to assume yes for an HTTP authentication type, as the
+      case OAUTH:
         // only way in is through some external system that the admin trusts
         //
         return true;