directory for testing formpack reports

  to the methods that use them because they are not
  reused very much, and hide the transformation that
  goes into building the Fields, Sections, and Versions
* added a `src` parameter to many of the fields / sections
  to facilitate debugging the content that was used
  to create the python objects in the form `version.Version`
* added a `pull.py` script to expose options to the CLI
* `select_one_external` is now the preferred alias throughout
  the code (underscores instead of spaces)
* added comment about xform_tools.normalize_data_type (obsolete)
  with `replace_aliases`, which is run on the content
  before the version is initialized.

…data as tests

with a method "run" that receives the formpack object

* strip "content" field from asset.json (redundant
  with versions json stored separately)
* use `date_deployed` rather than `date` (for now)

* at first, this will be used in just tests; eventually
  it could be a way to store the submissions that fed
  to the formpack via generators.

  translated_item
* passing TranslatedItem() object instead of dict
  when fields and sections are created
* added assertions which, when uncommented, should
  not fail (but don't want to risk AssertionErrors
  in production)

  TranslatedItem

(pack.submissions will be used in future tests)