Generate debug symbols for release builds

In order to make crash logs more useful to us, we should be using dSYM files and re-symbolicating (Apple's word, not mine!) our crash logs so we get pretty methods instead of just numbers.

This is pretty easy with Xcode 4:

* Instead of 'build' choose 'Archive' from the product menu
* An archive gets plonked in ~/Library/Developer/Xcode/Archives
* If you want to re-symbolicate a crash report: make sure you have the archive (Rob or whoever's building may need to send these out), then go to the organiser in Xcode( ⌘⇧2) click 'Devices', then 'Device Logs' and import your crash report and be done

Command line builds: just append 'archive' to the [end of your command](http://stackoverflow.com/questions/5384999/archive-with-xcodes-command-line-build-tool-xcodebuild-archive#answer-10306937)