Events subscription

[dzdidi]

Allow (via web-sockets?) to subscribe to events:

• payment state update
• channel state update
• forward state update
• invoice state update
• request state update
• peer state update

The more gradual events the better, for example in case of channel possible state transition could be:
openning -> open -> closing -> closed
as each state update can have specific business logic attached to it

[G8XSU]

This should be feature complete after #51, #53, #57.
We can always publish more events in the future but for now the bare minimum & required payment events should be published.

[G8XSU]

Marking this as closed, we can always add more events but for now the bare minimum & required payment events should be published.