Add Podfile to install RouteGuide proto library in sample app

objective-c/route_guide/Podfile

+source 'https://github.com/CocoaPods/Specs.git'
+platform :ios, '8.0'
+
+target 'RouteGuideClient' do
+  # Depend on the generated RouteGuide library.
+  pod 'RouteGuide', :path => '.'
+end