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.
Rename EphemeralFileNode to SessionFileNode.

From the DataTree point of view there's no ephemeral file, it's the
Pulsed's responsibility to maintain which sessions are online which
are offline and explicitly delete all the nodes of a session who is
offline.
Implement session file in DataTree.

- User can create a file of a given session.
- User can delete all the files of a given session.
stuff