Value normalizer, composer.json (#17) and fixtures update #18

In this PR some not linked subjects :

 - Creation of the composer.json file
 - Fixtures update for the Magento demo data
 - ProductNormalizer splitted in ProductNormalizer and ValueNormalizer

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [yes]
Checkstyle issues: [no]