HOWTO zip project to a file for deployment

Modify build.sbt to add command `sbt/sbt zip`:

``` scala
val zip = taskKey[Unit]("zip")

// Run xitrum-package before zipping
zip <<= (XitrumPackage.xitrumPackageKey, name, version) map { (_, n, v) =>
  // Zip directory target/xitrum to <project name>-<version>.zip
  val zipFile = n + "-" + v + ".zip"
  println(zipFile)
}
```

See also: http://jsuereth.com/scala/2013/06/11/effective-sbt.html
