Adding attrs arg to create_group and create_dataset

As it stands now, when one writes a group or dataset, one must write the attributes as a separate operation. This results in creation of .zattrs once. Then if one update the attributes, it results in writing .zattrs again. With something like a ZipStore backed Zarr Group, one gets a warning on writing .zattrs the second time, which AFAICT is unavoidable unless someone sets the attributes correctly the first time. So if we include an argument for attrs to create_group and create_dataset, we should be able to avoid this warning by setting the attributes during creation of these Zarr objects.

[alimanfoo]

Thinking about this some more, I actually think the current behaviour is a bit pointless: when an array or group is created, an empty dict is written to the .zattrs key, but there is no need to write .zattrs at all unless the user actually wants to store some attributes. I’m tempted to change this so that .zattrs is only written when the user actually wants to store some attributes. This would obviate the need for an ‘attrs’ argument to array or group creation, because, e.g.:

g = zarr.group()
g.attrs.update(foo='bar', baz='qux')

…would then imply only a single write to .zattrs.

[alimanfoo]

I’ve gone ahead and implemented this in #200, everything works fine and should be fully backwards compatible with code and data, so seems like a good thing to me.