Product Variation Order Again Bug Fix and Improvement

* Fixed a bug introduced with the new order item meta data structure of
WC 2.0: product variations could not be "ordered again"
* Improved the order again support for products with custom item meta
data
Product Stock Filters

Added a number of filters related to product stock to allow for improved
control over the stock handling and display
Merge pull request #2748 from justinstern/master

Product Variations Order Again Bug Fix and Stock Quantity Filters