[BUILD] Build script for the IntelliJ plugin

The build.xml was originally generated by IntelliJ, and then
restructured and beautified by Graham Allan.
Further contributions by: Arndt Lasarzik and Franz Zieris.

Change-Id: Iebd8a7eca6f9edfefaddf4765b5897b55b2f9a76
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1476
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>