Admin websocket refactoring

[FZambia]

For #73

[FZambia]

To summarize what have been done here:

New option admin - it enables admin socket which is now off by default. For backwards compatibility web option also enables admin socket automatically (because web uses this socket).

So to start Centrifugo with admin socket enabled:

centrifugo --config=config.json --admin

But this will also enable admin socket handler (and additionally will serve web interface):

centrifugo --config=config.json --web

Also --insecure_web is now --insecure_admin but old option still supported to not break things.

web_password and web_secret are now admin_password and admin_secret but old option names supported as well.

Also web interface can now use only AuthHandler and admin websocket for everything - so I removed InfoHandler and ActionHandler. Refactored web interface frontend at moment sits in its own branch in web repo - will embed it in the end.

Also the work here is not complete because I think it will be good to add message queue for admin connection (like client's message queue) - but this does not affect old functionality so can be done later.

[banks]

Didn't look in great detail but overall it seems good.