Add Kiwi for unit tests.

- Add unit test target "CocoaPodsTests".
- Add Podfile, specifying Kiwi as a dependency for the test target.
- Add CocoaPods.xcworkspace, which includes Pods.
- Configure test target settings: bundle loader, test host, etc.
- Remove main target as a dependency for the test target because
  OCUnit could not find main target, instead compile main target
  source files during test target build phase.
- Add test for CocoaPods +docsetInstallPath.

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This is a first step for adding test coverage to this project. See https://github.com/mneorr/Alcatraz for a great example of unit testing Xcode plugins.