New command to preload Scala on a device/emulator

Here is a new command to quickly setup your device (or emulator) for Scala development.

It predexes the Scala libs then pushes them to the device, most of it being heavily inspired by [svenwiegand/scala-libs-for-android-emulator](https://github.com/svenwiegand/scala-libs-for-android-emulator). These are the two main commands you can use :

  * `android:preload-device`
  * `android:unload-device`

I tested it on my device (Galaxy Nexus running Cyanogen 10.1-M2) and it apparently runs flawlessly.