Decouple channel rules from library core

[FZambia]

Relates to #128

This should make library less opinionated and give more freedom for developers in terms of implementing their own permissions and channel rules.

This is а big refactoring, which should make library more general and extensible. What's inside:

1. Remove opinionated layer: channel namespaces, channel rules (like user-limited channel, private channels), JWT parsing
2. Leave a possibility to implement the same with some code on top of library
3. Make the library feel as wrapper on top of client-server protocol
4. Event handlers should be set for connection to enable subscribe, publish, history, presence and presence stats. Otherwise client will receive Not Available error
5. Node config function to let developer return per-channel ChannelOptions to enable features, most of ChannelOptions removed (will be part of Centrifugo server), only core things left here.

Some personal thoughts: I always wanted to remove opinionated layer from library, this pr is a big shift and I am sorry for breaking things. But since we don't have v1 release yet I feel I have a right to do what I feel is a right choice in library positioning. This also should resolve issues like #113 and #80 – otherwise issues like this will always appear making library harder to support with time.