Feature - (2.0.0-beta) 'node' connection mode

[lifely]

Description

status: last testing stage

This might looks like a lots of change but the pull-request include differents things:

• new files and update to projects / settings (.gitignore, pod spec, example project, playground)
• some reoganisation of extensions into new files or re-order of properties of functions
• update multiple types with news attributes for feature like 'security', 'discoveryInfo' & 'connectionMode'
• update of the EventProducers for Sessions, Advertiser & Browser

Sessions - MCSession delegate & event producer

PeerSession can represent either a advertiser session (local-service) or a connection to a 'distant-service' invited through PeerBrowser

thus we now have access to the following attributes:

• sessionPeer - indicate this is a distance-service-session and represent the peer advertising that servicer
• isLocalServiceSession - computed property that check peer against peerSession
• isDistantServiceSession - computed property too

PeerSession can now take security parameter at init allowing to configurer the kind of security to use on the MCSession

Advertisers - MCNearbyServiceAdvertiser delegate & event producer

Advertiser didn't change too much, we can now use discoverInfo at init which couple with Peer.context allow better introspection of services and peer.

Before we used poorly named service to differentiate environment or restaurants, now we can name our services simply and use discoveryInfo to filter matching services.

Browser - MCNearbyServiceBrowser delegate & event producer

Main change to browser is the new sessionFactory logic introduced.
The browser will use the sessions attribute when uses as a single slave connection,
but will create new session through the factory if configured as node browser.

This will allow to create new session for each services we're connecting to.

[rchatham]

@lifely This looks super interesting! Very interested and thank you for contributing!