Add easy way to disable inventory tracking.

Currently inventory is always being tracked (resulting in backorders if not replenished).
This PR adds a simple NoopInventoryOperator and a configuration switch.
Merge pull request #854 from Richtermeister/inventory-noop

Add easy way to disable inventory tracking.
Update README for more specific behat setup rules

Include the database and schema creation
and add phpcr initalization note
Merge pull request #919 from mykehsd/feature/BehatDocumentation

Documentation enhancements for Behat setup
Include PHPUnit since it's not required by behat to run, but we're using it in WebBundle/Behat/WebUser.php
Merge pull request #918 from mykehsd/ComposerPHPUnit-Fix

Composer php unit fix
[OrderBundle] Item & order cost can't be negative