Add basic init subcommand that just generates an empty Podfile

lib/cocoapods/command/init.rb

+module Pod
+  class Command
+    class Init < Command
+      self.summary = 'Create a Podfile'
+
+      self.description = <<-DESC
+        Creates a Podfile for the current directory if none currently exists.
+      DESC
+
+      def run
+        (Pathname.pwd + "Podfile").open('w') { |f| f << '' }
+      end
+    end
+  end
+end