Refactor Chunks

Rename 

    Chunks -> ChunkIterable
    ChunkIter -> ChunkIterator

To more closely match the names in `collections`

Also add 

    ChunkIndexable

I believe that chunks might effectively replace `Concat` for many use cases, in particular `into` implementations on many data descriptors (cc @talumbau).  This abstraction is much lighter-weight than the old `Concat`.

Chunk Indexable is particularly useful for parallelism in which we'd like to give each process the ability to pull of a particular subset of the data.  I may play with this with BColz + Pandas.  It would also be relevant for multiprocessing and CSV loading.