Introduce sparkle command

This PR represents a substantial overhaul of sparkle's build system:

Instead of using maven to build .java source files, we use Gradle, which the Android ecosystem tends to prefer. The benefit for us is a substantially smaller and easier to understand non XML-based build description.
Instead of building these class files as part of creating a new app, we build them once and for all as part of the sparkle package build and install the resulting .jar in a known location as a data-file.
Building an app no longer involves calling maven from within a source checkout of sparkle: you can now use the new sparkle command for the packaging step.

So to build hello-world:

$ stack build
$ stack exec sparkle package sparkle-example-hello

That produces a .jar file in the current directory suitable for use with spark-submit.

Fixes #33.

cc @alanz could you try this out and let us know whether this works for you? I'd assign you as a reviewer but Github won't let me... :-/

tweag / sparkle