Refactoring streamcorpus_elasticsearch into a streamcorpus_pipeline
writer stage.

The most important code determines the fields that get indexed:
streamcorpus_elasticsearch/indexing.py

It uses an entry point to automatically add itself to the available
stages in streamcorpus_pipeline, so you can use it simply by following
these steps:

  ## install everything from pypi
  pip install streamcorpus_elasticsearch

  ## point to some KBA corpus paths in S3 as test data
  ## this requires that you have appropriate keys for s3 and decrypting this TREC corpus
  echo '
2011-10-07-14/social-500-00072b0eb141b558428180c93096f509-fdb9a342aac81e7a2189de8e9d7f57b4.sc.xz.gpg
2011-10-07-14/social-500-031ed0cda2f754f0dc8294c020a0052d-e6ca047022627094aa343bb2443cd140.sc.xz.gpg
2011-10-07-14/social-500-039d33f4638a7ec8a97718a79c2b2018-1d97da45fbef0c39a1c1fd86677384c7.sc.xz.gpg
2011-10-07-14/social-500-0634329703f20fc5e42aa07efcce67bf-d0d642c746cf25a1dd0188b5629584d9.sc.xz.gpg
2011-10-07-14/social-500-07af9fc936e19d5c645f0097cc1d530a-c45186d761517a6cc6534e5bce534294.sc.xz.gpg
2011-10-07-14/social-500-0e3495d6b130620082fc4d2a658b4216-b123ba5f1553215cac9807af1539c34c.sc.xz.gpg
2011-10-07-14/social-500-12c9e34c3f71c7ed8ac15bfbc9b320bc-178168921bd41eb46c3bb0399b31a8c8.sc.xz.gpg
2011-10-07-14/social-500-175f73776e2bf466fd2de14d702cf202-e81fdb9f125eb8a8ef0a014619afd390.sc.xz.gpg
2011-10-07-14/social-500-1803425b19f6acea76ea51f408bb962e-63b08b97e4c6ccd63b50f07e839dcc79.sc.xz.gpg
2011-10-07-14/social-500-1a11e20eeccea411030c1ba815df94d6-2414728697a80733be6f170dcb863b20.sc.xz.gpg
' > s3_paths.txt

  ## REQUIRED: update configs/kba2014-load.yaml with your
  ## elasticsearch cluster network names

  ## OPTIONAL: update configs/kba2014-load.yaml with a better index
  ## name than the example

  ## run the pipeline
  streamcorpus_pipeline -c configs/kba2014-load.yaml -f s3_paths.txt

  ## to clean things up, you can delete the data from that test run
  python -m streamcorpus_elasticsearch.manage -c configs/kba2014-load.yaml  stream-items-index
fixing package name to be consistent with other streamcorpus_* things