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.
Add unit tests for menu item creation.

- Add menu item tests.
- Add test helper code to load plugin, create an
  application menu object.
- Reveal -doesPodfileExist method in spec file. Ideally, this
  method would be defined on a separate class responsible
  for providing information on the current state of the
  workspace.
Merge pull request #4 from modocache/add-unit-tests

Add Kiwi for unit tests.