lib/cluster-store.js: improved API docs

Marked most members as private, polished text of documentation comments.

Renamed _setup() to setup() so that it can be documented.

Fixed code blocks in README.md to be compatible with strong-docs syntax
highlighing (renamed `Javascript` to `js`, `Shell` to `sh`).
Merge pull request #4 from strongloop/docfix

lib/cluster-store.js: improved API docs