* Appends values to one or more columns within a single row.
* This operation guaranteed atomicity to readers. Appends are done
* under a single row lock, so write operations to a row are synchronized, and
* readers are guaranteed to see this operation fully completed.
* @param append object that specifies the columns and amounts to be used
* for the increment operations
* @throws IOException e
* @return values of columns after the append operation (maybe null)