Bug-fix and improvements (Tested on Magento 1.9.x)

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.