Store result ids on ImportResult (postgres only)

[saulm]

I was looking for a solution to #179

Luckily I'm using postgres, and activerecord-import stores the ids for postgres: https://github.com/zdennis/activerecord-import#return-info

I understand this is not the ideal solution for everyone, but it's consistent with the behavior of the underlying base library.

Let me know if this is something you would like to add or if you need any other changes.

[coveralls]

Coverage decreased (-0.006%) to 97.98% when pulling 54fb15b on polyglot-solutions:master into 1c7601f on activeadmin-plugins:master.