First useful sbt configuration to develop for my Pi

Deploying Java libraries to a mounted sftp directory already works.

Creating JNI .h files works too.

Things to do next:

- create a proper sbt plugin for the PiDev sbt build class.

- use "sshj" library to access the Pi over ssh. This has to be
  executable on Windows.

- automate compiling the shared library on the Pi. It is small enough to
  compile directly there. Cross-compilation on Windows is just too
  complicated.