make the decorators work with both @classmethod and @staticmethod

[Ducky2048]

This should fix issue #21

[ro70]

@Peroxid I tried your fixes and it does not seem to work together with a second decorator.

@classmethod
@authenticated_users
def has_destroy_permission(cls, request):
   ...    

The error thrown is

File "/core/src/dry-rest-permissions/dry_rest_permissions/generics.py", line 262, in to_representation
results[action] = getattr(self.parent.Meta.model, method_names['global'])(self.context['request'])
File "/core/src/dry-rest-permissions/dry_rest_permissions/generics.py", line 306, in func_wrapper
if not(request.user and request.user.is_authenticated):
AttributeError: type object '...' has no attribute 'user'

[ro70]

@Peroxid Why not use something like

if args[0].__name__ == 'Request':
            request = args[0]
        else:
            request = args[1]

as proposed by @jjlorenzo in #21