Improve support for OS X command line tools

Following #193, in OS X linking against AppKit should be optional:

* Avoid relying in AppKit to check OS X versions.

As for TTY logger need for NSColor, I think the solution would be to create a B/W or CLI-compatible TTY logger, which could be in turn the supper class of our current TTY logger.

Also a subspec `CocoaLumbjerack/CLI` would avoid compiling non-CLI-friendly code.

Please give the branch a try and if you want you can ask for permissions to commit directly to it, as I don't actually develop much for OS X at the moment.