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.
Add Customer Welcome Email
Setting updatedAt in constructor and updating getUpdatedAt method to use updatedAt var
Removed the setting of updatedAt from the constructor
Set updatedAt to be nullable in the doctrine mapping
Setting createdAt in constructor
Merge pull request #892 from pfwd/user-bug

Setting updatedAt in constructor and updating getUpdatedAt method to use  updatedAt var
Added a constructor and setting createdAt
Merge pull request #896 from pfwd/promotion-bug

Setting createdAt in constructor
Merge pull request #898 from pfwd/exchange-bug

Added a constructor and setting createdAt
Optimize cart totals retrieving from menu builder
Nesting email settings & configuration
Issue #895 taxon selection optimization
Merge pull request #901 from umpirsky/fix/issue-895-taxon

Issue #895 taxon selection optimization
Fix VariantCombinationValidator and backend variants display to work with products without options.
Fix template condition to keep tests passing.
Update Payum requirements in PayumBundle
Removing address forms from backend user form
Removing outdated scenario step.
Merge pull request #890 from Richtermeister/admin-create-customer

Removing address forms from backend user form
Merge pull request #900 from winzou/frontend/menu

Optimize cart totals retrieving from menu builder