#277: Add support for generating an heroku system.properties

This change adds support for a generating a `system.properties` file in an uploaded project.  This allows one to configure the heroku job to use a different Java runtime version.  By specifying this as a generic property, in the future any other properties may be cleanly added.

While I created a unit test to assert that it was working properly, when I attempted to run `mvn clean install -Pqulice`, I ran into this unrelated warning:

```
org.apache.maven.artifact.factory.ArtifactFactory in org.apache.maven.artifact.factory has been deprecated
```

Since `-Werror` has been specified for this project the entire build fails.  I'm not sure what to supercede `ArtifactFactory` with, and since this is unrelated to what I was working on I at least wanted to put the rest of the change I had done in front of you.

Let me know if there's anything I can do to move this further along.

Thanks