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
Nesting email settings & configuration
Fix VariantCombinationValidator and backend variants display to work with products without options.
Fix template condition to keep tests passing.
Merge pull request #903 from Richtermeister/product-without-options

Fix variant validation and backend display for products without options.
Merge pull request #878 from Richtermeister/customer-welcome

Add Customer Welcome Email
[TaxonomiesBundle] Cleanup models to match latest changes
Reduce Travis-CI build on main repository to only master branch
Translate address stuff
Translate personal home stuff
Translate order stuff
Translate general stuff, blocks and breadcrumbs
Translate exchange rates
Translate frontend stuff
Translate payments and promotions
Translate taxonomies
Translate user stuff
Translate menu and sidebar
Translate messages to German