Foundational concept design: multi-node support

Praefectus should support working with multiple nodes. There'll be multiple node from the very beginning: there'll be a Taskwarrior node (with import/export capabilities), and a Praefectus user-facing node (so the user may customize/change the tasks imported from Taskwarrior, with the changes eventually being synchronized back to Taskwarrior).

Here're the questions we'll need to solve while designing this stuff:
- should task attributes belong to individual nodes and/or to node types and/or to the global namespace?
- how the attributes should be merged if multiple nodes have changed the same attribute?
- should we provide a pluggable/configurable merge algorithms?

Depends on:
- [ ] #6 (since we need to decide on the sorts of information we store in Praefectus first)
- [ ] #7 (since we need to know how the branching/merging will work before applying it to the multi-node practice)