Merge pull request #9 from umpirsky/composer-stability

Fixed minimum-stability in composer.json
Inventory backorders.
Merge pull request #10 from umpirsky/inventory-backorders

Inventory backorders support.
Added InventoryChangeListener
Merge pull request #11 from umpirsky/inventory-listener

Added InventoryChangeListener
Doc tweaks
Merge pull request #12 from umpirsky/doc-fixes

Doc tweaks
CS fixes
Added default Stockable model
Update .travis.yml
Merge pull request #14 from stloyd/patch-1

Update .travis.yml
Merge pull request #13 from umpirsky/stockable-model

Added default Stockable model