Configured travis-ci build.
Implemented ClusterStore.

- pub/sub part (handshake in cluster)
- per-client store (key-value store shared in cluster)
Improved documentation.

- Reworked README.md to have the same structure as cluster-connect-store.
- Added travis-ci badge and npm badge.
- Added jsdoc comments to source files
- Fixed jshint warning about using == instead of ===
- Renamed Store to ClusterStore in the unit-test to be consistent with docs.