Remove compile directory if it exists before downloading source

Fix annoying behavior where, after a failed compilation, it is needed to run buildout to have the compile-directory removed but have an error message, then run it again to try to compile again.

Update tests to show this wanted behavior.

Also include a .gitignore file, as a bonus.

Maybe old behavior is a planned behavior, but in this case, the code is anyway not valid, because the directory will be anyway deleted after one buildout run.