Add Factset

- Factset contains a FactsetId and list of partitions
- Add more tests to PartitionSpec
Had a chat with @markhibberd and we are removing `NamespacePartitions` in favor of just `Partitions` which is a list of `Partition` (see [comment](https://github.com/ambiata/ivory/pull/192#issuecomment-51753815) below)