Revert changes made to split and cross build, change attachment to Map[String, Any]

Since adding a new library in SBT seems very difficult, this PR reverts the changes made to split and cross build scalahost (all commits since 53d7d47f09b55ef6cac19b2355a0e2325e86e158), and changes the type of the attachment to be simply a map from String to Any.

To extract the attachments, one now simply needs to cast the values of this map.

[This diff shows that the revert is ok, right ?](https://github.com/Duhemm/scalahost/compare/scalareflect:53d7d47f09b55ef6cac19b2355a0e2325e86e158...HEAD~1)