Write the code signing identity specified via C.CodeSign into the Xcode ...

Xcode 5 will by default sign the .app file that JamPlus makes with the signing identity specified in the workspace. Previously this would always be 'iPhone Developer' however the C.CodeSign rules can set this to anything (and you need to use a different setting per project and per configuration of development vs adhoc vs appstore). This pull request writes the identify from C.CodeSign into the workspace so Xcode will sign correctly.

Note: ideally Xcode wouldn't code sign at all (as JamPlus handles all of this). This behaviour is possible, however it requires a modification to an Xcode plist file. I'll go into this in a separate pull request.