Scope Policy functionality

In RFC6749' we can read « If the client omits the scope parameter when requesting authorization, the authorization server MUST either process the request using a pre-defined default value or fail the request
indicating an invalid scope. »

This PR add this rule.
Supported scopes, default scopes and scope policy can be defined per client (eg: limitation of supported scope depending on the client's type).

New parameters have been added:
- CONFIG_SCOPES_POLICY: set the policy of the server (either error or default)
- CONFIG_DEFAULT_SCOPES: if policy is default, this array of scopes will be used as default.

This PR also fix bug #25

**It changes the client interface, so that this PR breaks compatibility and requires to bump version number.**