Sync product attributes

[heshamMassoud]

Implementation details

1. Product types should be fetched and cached on-demand per batch.
2. Only some, but not the entire, information on Product types are needed to be fetched:
   a. Each attribute's type: The types that we should be concerned with for the MVP are BooleanType, TextType, LocalizableTextType, EnumType, LocalizableEnumType, NumberType and SetType.
   b. The EnumType and LocalizableEnumType values should not be cached since they are big and are not needed.
   c. isRequired field: if the field is required but doesn't actually have a new value, the sync library would trigger the error call back instead of making a request to CTP.
   d. attributeConstraint field: if it has the value SameForAll, then the setAttributeInAllVariants update action should be used for adding/removing/changing an attribute value at the same time for all the variants.

P.S. The name of every attribute is unique.

Hints

Product#masterData#staged#variants#attributes

http://dev.commercetools.com/http-api-projects-products.html#attribute

Update Actions required

1. addVariant
2. removeVariant
3. changeMasterVariant
4. setSku
5. setAttribute
6. setAttributeInAllVariants
7. setProductVariantKey

[ahmetoz]

The issue looks like not managed after it's open. The changed described in the issue is already implemented.