Define Importers::Csv to load seed data

  * Importers::Csv loads specifically formatted csv data
  * One row in a CSV file will generally map to one model in the datbase, but a
    row could also map to a previously existing model.  Determine if a row or
    any of its data maps to a previously existing model by defining unique
    columns in the csv.  Add data to previously existing models as appropriate.
  * Images will not be associated with items if the image file path is not
    specifed, and the image file path must be on the local file system.
  * Add sample_data.csv and images associated with the items as seed data
  * Seed data will be loaded by rake db:seed using Importers::Csv

Add PartialDate to store/retrieve model partial dates

  * Dates will be stored as integers in the database anywhere parital dates are
    needed
  * Override default attribute accessor readers for models with partial dates to
    return new ParitalDate::Date instance

Update Location#current uniqueness validation

  * Make the validation conditional to account for item_ids that are nil since
    the validation is scoped to item_id