Bug-fixes and improvements related to attribute import and XML structure changes

1. Fixed - Existing product-product relations (associations, related,
cross-sell, up-sell) were being over-written when updates containing
subset of relations were being imported.
2. Updated XML schema to better enclose attributes, attributegroups and
attributesets.
3. Fixed import of text and textarea attributes for simple and
configurable products and modified attribute value definitions
accordingly.
4. Implemented import of position value for attribute options values.
5. Fixed import of non-configurable attributes for configurable
products.

Author: intelliant01