Refactor to have ckanclient.CatalogClient and ckanclient.DataStoreClient

[seanh]

Want to change:

import ckanclient -> import ckanclient.CatalogClient import datastore -> import ckanclient.DataStoreClient

Move stuff out of ckanclient/__init__.py into catalog.py:

ckanclient/
  catalog.py
  datastore.py

Maybe also do the samething with the FileStore, i.e. have ckanclient.FileStoreClient.

What to do about contrib and loaders? Anything?