create `podspec` file for cocoapods.

With the file, we can integrate `react-native-picker` with an exist native `iOS` project like bellow:
``` ruby
// Podfile
pod 'Picker', :path => '../YeSanPoRN/node_modules/react-native-picker'
```

Picker.podspec

+require 'json'
+
+package = JSON.parse(File.read(File.join(__dir__, 'package.json')))
+
+Pod::Spec.new do |s|
+
+  s.name           = 'Picker'
+  s.version        = package['version'].gsub(/v|-beta/, '')
+  s.summary        = package['description']
+  s.author         = package['author']
+  s.license        = package['license']
+  s.homepage       = package['homepage']
+  s.source         = { :git => 'https://github.com/beefe/react-native-picker.git', :tag => "v#{s.version}"}
+  s.platform       = :ios, '7.0'
+  s.preserve_paths = '*.js'
+
+  s.dependency 'React'
+
+  s.subspec 'Core' do |ss|
+    ss.source_files = 'ios/RCTBEEPickerManager/*.{h,m}'
+    ss.public_header_files = ['ios/RCTBEEPickerManager/*.h']
+  end
+
+end