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 think it might be even better to do the handling of invalid lines in the DelimitedInputFormat as it delegates the parsing of lines and could read the next line if something fails.