Add Realtime::Channels plus support attach & detach, and releasing Channel resources

- Added Realtime::Channels collection to Realtime client library, plus share functionality of Channel Collections with a module
- Remove unused Client#tls attribute, force use of #use_tls?
- Improved and more consistent documentation throughout 
- Support channel options for Realtime channels
- Delegate Realtime::Client#stats & Realtime::Client#time to Rest::Client\
- Support callback blocks for Channel#attach and #Channel#detach