Upgrade PeerTalk project structure for Swift Package Manager

[programmarchy]

Hello! I've got a big, opinionated, but hopefully useful PR, which does the following:

• Adds support for Swift Package Manager
• Consistently applies "PeerTalk" spelling (not "Peertalk" or "peertalk")
• Fixes all warnings and upgrades projects files to latest Xcode
• Configures tests for both iOS and macOS
• Includes sample entitlements file for sandboxed macOS app
• Moves examples into separate directory and xcworkspace
• Bumps version to 2.0 (since this will be a breaking change)

This PR does not make any code changes, except the minimum necessary to play well with Swift Package Manager.

PR #56 got the ball rolling for Swift Package Manager, but it wasn't fully complete. So I continued that work, using the AFNetworking repo as a guide, since it supports Swift Package Manager, CocoaPods, a standard Framework target, and Carthage.

Here's roughly the steps that I took:

• Renamed peertalk to Sources
• Created Package.swift
• Changed header imports to double quoted instead of angle bracketed (required by SPM)
• Created two separate umbrella headers:
  • Sources/PeerTalk.h for SPM with double quoted headers
  • Framework/PeerTalk.h for use in the standard Framework targets
• Moved Info.plist to Framework for use by standard targets and so it's not picked up by SPM
• Renamed peertalk-tests to Tests and configured targets for both iOS and macOS
• Performed recommended Xcode upgrades
• Suppressed a warning CLANG_WARN_QUOTED_INCLUDE_IN_FRAMEWORK_HEADER
• Moved example projects into Example folder and created a separate xcproject and xcworkspace to house them
• Regenerated the icons from the PDF in the etc folder
• Searched and replaced i.e. s/Peertalk|peertalk/PeerTalk/g
• Updated podspec to pick up source files in new path location
• Deleted some apparently unused items (index.html and etc/styles.css)

Phew! It's a lot but I think this project was due for a spring cleaning.

I've tested the following:

• Swift Package
• Unit tests passing
• macOS and iOS apps run and talk to each other
• Building framework

But haven't tested CocoaPods or Carthage yet.

[programmarchy]

Requesting review from @rsms @jonathandann

Also @tonyarnold @hughbe @sieren who also wanted Swift Package support