Partition name change

In order to address the limitations around partitioning and the hive integration, the readme has more details. I have changed the partition code to prefix columns by `partition_` where the partition column is directly derived from a thrift field.

I also added documentation to Partition.
Done. Thanks for the remainder.

It is used to inform how scalding should write out the files on disk by splitting them into different folders. It is also used to configure partitioning in hive.