replace DSet with DColl (fixes #53)

- replace DSet (the Datomic type for sets) with DColl (the Datomic type for more generally collections).

- DColl is a scala.collection.Iterable under the hood: it will take java.util.Collection data from Datomic and the type classes for casting and reading/writing attributes will treat it with set semantics.

- also replace Datomic.set with Datomic.coll

- fix up tests
minor: simplify DWrapper

appears to be no need to separate trait and case class… just a simple class will do
minor: fix doc string
Revert "minor: simplify DWrapper"

This reverts commit 1c4a5bc8c41cfac96eb099dd84bb1fa4bdc4e003.
Merge pull request #57 from pellucidanalytics/topic/DColl

Replace DSet with DColl as the type that wraps all collections that go to and from Datomic.