Fail installation of Cedar framework when Xcode fails to copy headers.

Before this was silently passing, but it tell the user to reinstall
since this may occationally fail because of Xcode.
Merge pull request #274 from jeffh/fail-framework-if-no-headers

Fail installation of Cedar framework when Xcode fails to copy headers.
Updated README to include an example
Merge pull request #278 from jeffh/example-in-readme

Updated README to include an example.