Websocket support

[nikolap]

Goals with this feature addition. If any are misaligned with the lib, please push back.

1. Allow user to override any of the implementations, e.g. custom listener or callback
2. Attempt to keep API similar (though not identical) to immutant -- should allow for relatively easy plugability with libs like sente
3. Make modular, i.e. so that it can be moved as needed and not directly tied into pohjavirta. The rationale is that many web applications do not require this so it should be an opt-in functionality with custom user-written code to hook it together (perhaps should add helpers?)

Open to feedback and discussion before merging.

Re #1

[ikitommi]

Looking good, thanks!!