Let the application determine allowed scopes based on client

Changes:

OAuth2_Storage_ScopeInterface:
getSupportedScopes now takes one parameter: $client_id.

OAuth2_Storage_Memory:
getSupportedScopes takes its required parameter, but doesn't do anything
with it.

OAuth2_Scope:
getSupportedScopes passes its parameter on to the storage object.

OAuth2_Controller_AuthorizeController:
handleAuthorizeRequest passes on its $client_id variable when it calls
validateAuthorizeRequest.

ScopeTest:
Adds dummy parameter to the getSupportedScopes method call.
Update readme file to reflect new getSupportedScopes API
Merge pull request #66 from AdamZ/scope-customization

Scope customization