Download boilerplate APKs for Droidium by Spacelift

Ftesting with Droidium native plugin requires selendroid-server.apk from Selendroid project. Web testing requires android-driver.apk from same project.

Right now, these apks need to be downloaded manually by user so he has to find it on the net and manually setup it in arq.xml. As this is not desired, Spacelift 1.0.0.Alpha2 can download these artifacts automatically and they can be downloaded to target/ when it is not found so user does not have to do a thing.

https://gist.github.com/kpiwko/10738725