Add attribute sets with their attribute groups import

Attribute set import and update
Attribute groups import, removing
Attribute groups sort order update seems not work
Add attributes import and delete
Improve generation of attributes and some CS
Clean code and php doc
Refactor and split code

Fix bug with called api method
Fix bug with group order
Remove groupToUpdate of _updateAttributeSetsAndGroups
Split importAttributeSet in attribute sets and attribute associations
Rename method with "Magento"
Revert some cs and php doc
Update readme
Merge pull request #1 from akeneo/enhancement/attributes

Enhancement/attributes