dataprovider: add custom command and/or HTTP notifications on users add, update and delete

This way custom logic can be implemented for example to create a UNIX user
as asked in #58

config/config_test.go

@@ -29,7 +29,7 @@ func TestLoadConfigTest(t *testing.T) {
 		t.Errorf("error loading httpd conf")
 	}
 	emptyProviderConf := dataprovider.Config{}
-	if config.GetProviderConf() == emptyProviderConf {
+	if config.GetProviderConf().Driver == emptyProviderConf.Driver {
 		t.Errorf("error loading provider conf")
 	}
 	emptySFTPDConf := sftpd.Configuration{}