Prevent fixture from creating an empty product.

Unable to create the Magento fixture Notice: Undefined index: sku in vendor/magetest/magento-behat-extension/src/MageTest/MagentoExtension/Fixture/Product.php line 70
Add created at and resolve specs
Merge pull request #56 from robjstanley/patch-3

Prevent fixture from creating an empty product.
Exception is thrown if sku is missing in data grid
Merge pull request #58 from MageTest/feature/created-at-fix

Add created at and resolve specs
Merge pull request #59 from MageTest/feature/exception-thrown-if-sku-is-missing

Exception is thrown if sku is missing in data grid
FixtureFactory now correctly called from MagentoContext
Merge pull request #60 from MageTest/feature/spec-magento-context-use-of-fixture-factory

FixtureFactory now correctly called from MagentoContext
Merge branch 'develop' into feature/Product-image-fixture
Move image asset processing into private method