CRI-1212: Integrate macOS changes (#92)

* Add support for macOS

- Add PlayerView for switching between UIKit and AppKit view class
- Remove imports of UIKit
- Replace UIView uses with PlayerView
- Add #if for cases that are not supported on macOS
- Update podspec for macOS specific configuration

* Add documentation for PlayerView

* Fix brace placement

* Lower macOS minimum deployment target

* Change type oif RegularPlayerView to PlayerView

* Updated parameters to support macOS and tvOS

* Switch to canImport() from os()

* Revert canImport() back to os() in on instance

PlayerKit.podspec

@@ -4,7 +4,7 @@ Pod::Spec.new do |s|
   s.summary          = 'A modular video player system.'
 
   s.description      = <<-DESC
-  PlayerKit is a modular video player system for iOS and tvOS.
+  PlayerKit is a modular video player system for iOS, tvOS, & macOS.
                        DESC
 
   s.homepage         = 'https://github.com/vimeo/PlayerKit'
@@ -14,6 +14,10 @@ Pod::Spec.new do |s|
 
   s.ios.deployment_target = '10.3'
   s.tvos.deployment_target = '10.0'
+  s.osx.deployment_target = '10.11'
+
+  s.ios.framework = 'UIKit'
+  s.osx.framework = 'AppKit'
 
   s.swift_version = "5.0"