Release v2.0.3

This release introduces preliminary support for NATS Key-Value storage.

Summary of my changes since PR #23 :
Core NATS

• added -trace to the method jet_stream to log all JetStream requests similarly to --trace in NATS CLI (remember to set -log_level to debug as well)
• added new functions nats::isotime_to_msec and nats::msec_to_isotime - basically working around limitations of clock that doesn't work with milliseconds

JetStream

• added a convenience method empty_kv_bucket to delete all messages in a bucket without deleting the bucket itself
• added -description to create_kv_bucket

Key-Value

• removed the -timeout option from methods history and keys (but I'll add a test checking what happens in case of disconnection)
• content of the bucket status dict is now documented in KvAPI
• removed unwatch - to stop watching, just destroy the kv_watcher object
• removed -idle_heartbeat from watch (might come back depending on how I implement #26 )
• key is now mandatory argument for watch. Use ">" to watch all keys

Note that this release is not production-ready, and is intended only for development. Once the open issues are solved, a proper v2.1 will be released in the near future.