Add a meta dictionary to models.

Addresses #2189. 
This issue is milestoned "future" but I need it now.
It has been discussed on the mailing list already and there was an agreement to add it.

Instead I'd rather add `meta=None` to all the `__init__`s in this module, which would be clearer I think, though admittedly tedious (I had to go through and do that everwhere for adding `model_set_axis`)