enable CSV Reader to ignore invalid lines like an empty line at the end and comments - FLINK-1208

Corresponding to issue FLINK-1208:
enable CSV Reader to ignore invalid lines like an empty line at the end, multiple header lines or comments without throwing an exception
I'd put this at least at INFO level or even at DEBUG. Logging all invalid lines can become quite expensive.
I'd also prefer to do this aggregated (count of invalid lines) at the end of each input split to reduce the logging effort.

Maybe you can override `open()`, call `super.open()` and log after that...