Git ignore generated avro classes

I want to suggest that we remove the auto-generated Avro classes from git source control. In this pull request I've added that directory (model-elements-base) to the .gitignore file and removed the files.

The Maven build process will still create the Java classes and leave them sitting there in Eclipse etc of course. So debugging / local development will be completely unaffected.

The only effects of this change should be
- good: no longer worry about spurious "changes" to the auto-generated files after each build
- bad: no longer able to browse the contents of the auto-generated files on the web on github (few months ago I thought this could be significant ... but I never did it once)