Introduce paperclip. Change importer to take an Import.
Refactor SalesImporter to SRP.

* Extract ImportFormatParser to handle parsing and mapping keys.
* Extract SaleLoader responsible for loading each sale record.
* Move aggregation and completion logic into Import.  Import also
  becomes the entry point for processing a file.
* Also cleans up the ImportsController a bit.
* The primary spec, sales_importer_spec has been moved to
* spec/behavior/parsing_and_importing_spec.rb