Support Eclipse style multi-module projects

[ltackmann]

I use Eclipse which does not handle nested multi-module projects very well (mostly it just does not work). So my multi module project layout must be like this

./     <--- this is the root folder pushed to Heroku
./.git                            
./project-a/build.sbt
./project-b/build.sbt
./project-web/project/Build.scala  <--- this is the root SBT project
./Procfile

This means that the root folder for Heroku (the .git root) is not the same as the root project folder (project-web). It would be very welcome if the xsbt-start-script-plugin could handle these situations so the generated start script would work correctly when run from the real root.