Inventory backorders.
Added InventoryChangeListener
Doc tweaks
CS fixes
Added default Stockable model
Update .travis.yml
Merge pull request #13 from umpirsky/stockable-model

Added default Stockable model
Use AvailabilityChecker in InventoryOperator, add InsufficientStockException
Sorted backorders filling
Fixed URI to documentation
Removed inventory prefix
Add dev branch alias
Use new interfaces from SyliusResourceBundle
MongoDB ODM support added
[Minor] Typo fixed
Symfony 2.2 and 2.3 versions enabled
Resource bundle version bump
[Minor] Version number typo fixed
Fixed typos
Initial Symfony 2.3 upgrade