Add scripts for DB migration

* groups_to_ogs.sql: transforms groups into orgs and assigns datasets to
  orgs

* users_to_members.py: adds users to the members table according to
  their old group roles so these are moved into the relevant
  organization
Add a .gitignore file
Add file for new plugins and first stub of the publishers plugin
Add first draft of IDatasetForm

Adds just the filetype and country fields and the form fields (probably
in the wrong place)

TODO: * add rest of fields from package_iati.py to the schemas
      * add rest of fields to form (single page)
      * add rest of needed helpers
Merged ckan-2.x