* date and datetime fields are now converted to Python datetime objects
  before executing transitions and converted back to ISO format when
  storing the loan record
* add request_start_date to be able to track the start and in the
  end of a PENDING loan (request)
* moves transitions decorators checks from `before` method to
 `execute` to ensure decorators are executed as first thing
* makes configurable to automatically assign the item when creating a
  request via `assign_item` param in the transitions
* adds marshmallow loader for REST endpoint
* adds support to ES7
* changes permissions to take advantage of invenio-access
* ES mappings are now matching schema fields
* Drops support for Python 2
* closes #111, #106, #76, #81