Add com.eed3si9n.sbt-unidoc plugin

This plugin allows to generate one ScalaDoc site for all projects
combined.
Merge branch 'master' into dev
Refactor Commit and add parents encryption keys

net.secloud.core.objects.Commit fields treeId and treeKey have been
replaced by the field tree of type TreeEntry. This allows easier
recursion start in later algorithms.

net.secloud.core.objects.Commit field parentIds has been replaces by the
field parents of type List[net.secloud.core.objects.CommitParent]. The
type CommitParent bundles the parents ID and the parents encryption key.