Convert sip-lab to a generic sip library

[MayamaTakeshi]

Currently sip-lab is just a testing tool so I never cared to test it as a long running process or as a server app.
But we might eventually need this.

For this it should:

• properly support event notification per entity like call, subscription, subscriber, account etc so that client code can set event handlers for them.
• add methods to permit to connect media sessions between them (conf bridge)
• allow access to audio/video device (as someone might want to use it to make actual call)

Right now, the library itself is agnostic: we have addon.cpp that is used to permit to interface with node.js.
But you can create your own interface for other languages.