Implemented the store.
benchmark: added tls-connect.js benchmark

Created a simple benchmark to measure how quickly we are able to create
TLS connections and compare ClusterStore's performance gain.
lib, test, benchmark: code cleanup + API change

Simplified module's API as suggested in code review.

Cleaned up cluster-store implementation, extracted duplicated Buffer
manipulation into encode/decode functions.
lib: added session expiration

Configured strong-store-cluster collection to expire stored items
after 300 seconds (openssl's default).
lib: added jsdoc comments, updated README.md.
lib,test: correct handling of multiple servers

Modified the API so that installSessionHandler() takes an optional
namespace parameter that can be used to distingiush between multiple
servers.

The namespace is used as a prefix for NativeStore keys.