Update SPEC.md

Add spec about /pulsed/servers.
Merge pull request #60 from EasonLiao/update_spec

Update SPEC.md
Stores cluster members under /pulsed/servers of tree.

- Stores cluster configuration in DataTree.
- Inspecting cluster members is like inspecting regular files.
- Add integration test.
Merge pull request #61 from EasonLiao/pulsed_servers

Stores cluster members under /pulsed/servers of tree.