update: Rewrite permissions

@sylvain Done, here's how it works:

• On djangoldp Model.Meta.
• permission_classes is not mandatory anymore if not provided it'll be LDPPermissions.
• By default, no one have any right.
• Model.Meta now have anonymous_perms, authenticated_perms, owner_perms
• anonymous_perms, authenticated_perms and owner_perms can have view, add, change, control, or delete
• They can also have inherit - that is activated by default. Owner inherit from Auth who inherit from Anons.

egs. :

class EveryoneCanRead(Model):
    class Meta:
        anonymous_perms = ['view']
# Because default owner & auth are inherit.

class NoOneCanUseMe(Model):
    class Meta:

class AuthOnly(Model):
    class Meta:
        anonymous_perms = []
        authenticated_perms = ['view', 'add']
        owner_perms = ['inherit', 'change', 'control', 'delete']

class Notifications(Model):
    class Meta:
        anonymous_perms = ['add']
        authenticated_perms = []
        owner_perms = ['view', 'change']

class OhNoOwnerCantRead(Model):
    class Meta:
        anonymous_perms = ['view']
        authenticated_perms = ['inherit', 'add']
        owner_perms = ['change', 'control', 'delete']
# I supposed this is bad, but we don't want to always inherit permissions..?

Also, you can still overload it if you need more precise permissions (Member of a project for example)

@bleme If you can take a look too.