Keychain helper error messages, support OSX10.6+

Updates to the keychain helper.

* Provides plain text error messages using `SecCopyErrorMessageString()`
* Hides the item not found error code inside the `key_exists_p()` function - this is just noise given the functions purpose.
* Added compile support for OSX10.6+ in `script/build-keychain-helper`

Added a dependency to `CoreFoundation/CFString` for friendlier error messages.

Deprecated changes.
* <del>Renames the tool from `script/Boxen` to `script/KeychainHelper`</del>
<del>NOTE - The included script/KeychainHelper file has been compiled on 10.7.</del>