Work with different non-file luigi targets

[meliache]

@Bilokin reported that he created a custom database-target found that he had to create a dummy self.path attribute and a dummy makedirs() method to get it to work, otherwise exceptions were encountered.

I was surprised, I had expected that as long as you don't use any b2luigi-specific output methods like self.add_to_output() are used and b2luigi is basically only used for working on remote batch systems, all b2luigi targets should be supported. But I haven't looked at this properly yet, so I'm not sure if this should be considered a bug in b2luigi. Probably should create a minimal example for reproducing that error first.