Support install from podfile (#232)

* Added podspec

* Added Firebase/Messaging to dependencies

* Update to correct ios platform version

react-native-fcm.podspec

+require "json"
+package = JSON.parse(File.read('package.json'))
+
+Pod::Spec.new do |s|
+  s.name          = package['name']
+  s.version       = package['version']
+  s.summary       = package['description']
+  s.author        = "Libin Lu"
+  s.license       = package['license']
+  s.requires_arc  = true
+  s.homepage      = "https://github.com/evollu/react-native-fcm"
+  s.source        = { :git => 'https://github.com/evollu/react-native-fcm.git' }
+  s.platform      = :ios, '8.0'
+  s.source_files  = "ios/*.{h,m}"
+
+  s.dependency "React"
+  s.dependency "Firebase/Messaging"
+end