Add #xcode_task, #xcode_test, #xcode_build, #xcode_archive, #xcode_export

I want own namespace and prerequisites.
By adding subclass of Rake::Task, Rakefile can be written as following:
```
desc "export"
xcode_export :export => [:archive] do |t|
  t.workspace = "Example.xcworkspace"
  t.scheme = "Example"
  t.configuration = "Release"
  t.signing_identity = "iPhone Distribution:…"
  t.build_dir = "build"
end

desc "archive"
xcode_archive :archive do |t|
  t.archivePath = File.join("build", "Example.xcarchive")
  t.exportPath = File.join("build", "Example.ipa")
  t.exportProvisioningProfile = "Ad_Hoc.mobileprovision"
end
```