Send Accept: header when requesting access token to ensure a JSON
response comes back (required for GitHub)
 - Also improved debug logging
Re-load credentials from the repository when a 403 response is received
from the upstream provider
Build against 4.2.d release
OAuth 2.0 return web script improvements
  - Web script can be used directly, no need to declare a new bean
  - Endpoint ID taken from web script path token
  - All other parameters are looked up from the connector descriptor
Allow OAuth 2.0 parameters to be declared in the endpoint descriptor
instead of the connector
  - Avoids the need to declare new connectors in each project
  - Parameters are accessible via the 'remote' root context object in
JavaScript, allowing client IDs to be injected into page components.
Declare a generic HttpOAuth2QueryStringConnector connector which can be
used by client projects
OAuth 2.0 connector fixes
  - Retrying requests after an initial 401/403 response now works
  - Application context is now always set correctly on the connector
instance