@slack/client v2.0.0

Refactors the library to javascript, adds a lot of tests and restructures it to improve maintainability and extend functionality.

• Creates two separate clients:
  • RTM; manages connection to Slack's RTM API, including reconnects
  • Web; provideas a callback interface to all of Slack's Web API endpoints
• Moves the memory data store implementation off the clients and into its own class
• Uncouples the model objects from the clients; model functions to send messages to channels etc are now accessed via the web and RTM client
• Moves the transport layer (websockets and HTTP) to a pluggable model, so that complex transports (through request proxies etc) can be handled
• Adds test coverage on most core functionality in the library