Shadow terminology

@raronson @charleso @maxpow4h @etorreborre @markhibberd 
Adding some new terminology around Shadow*
I get what you mean, but I think this needs a little more detail, explain by example, such as  whilst a Repository is abstracted over many different types of backing stores, a ShadowRepository is specialized to only those stores that we can perform batch operations on. Right now this is only HDFS, in the future it may include other FS... as well when we implement scalaz-stream or similar batch processes. But the primary point is that Shadow is where "work" happens, and the non-Shadow data types are where data sits at rest. And the Shadow type is specific enough to exclude the possibility of data being somewhere we can't do processing on.