Fix for issue - https://issues.cloudera.org/browse/CDK-473

- CSV file header used to generate avro schema can only be Alphanumeric ( as per computer science definition).
- Additional check added to make sure that csv header should strictly follow Alphanumeric standards.
Sorry to be late to this, but can you put this into a method in org.kitesdk.data.spi.Compatibility called checkFieldName (or similar) please?