little generalization to be able to use repositories with username/pwd, ...

...e.g. my.datomic.com

Hi,

I added some small changes, which would be nice to have added to the code base:

- at least for the dev server I needed the artifact resolving to my.datomic.com, a repository with username/pwd and so far the :repository key just was a set
-> I changed it internally to use maps as pomegranate/aether does and distinguish just in the multi-method between the :repository value being a set or a map
-> the second/other part of the change is in boot

Michael