Support for null valued columns in hadoop cql3 CqlRecordWriter.

With this patch you can insert/update null values to cql3 table in hadoop job.

Usage:
```java
CqlConfigHelper.setOutputCql(job.getConfiguration(), "UPDATE outputTableName");
job.getConfiguration().setStrings("cassandra.output.cqlColumns", new String[] {"column1", "column2", ...});
```

There is also an option ```cassandra.output.skipNullColumns```. When it is set to ```true```, null valued columns are not inserted into table (they are simply skipped).

WARNING: This patch is not fully compatible with original version, because columns are no longer specified in CQL statement, but they must be specified in separate parameter ```cassandra.output.cqlColumns```.