Feature/importexport

Hi guys,

I've added functionality for importing stock data with the ImportExport module. Changes include:

* New Location attribute called "code". Allows users to import data by specifying a location code
* ImportExport observer - preserves default ImportExport stock import functionality but will also import data for each location
* When importing a CSV, you can include a column called `stock_location` that defines which location to match the stock data to.
* Included sample CSV

See the updated README.md for more details.