Import category tree based on ID specified on website #2096 fixes #89

The category tree import might fail sometimes if no ID is provided. Explicitly
allow the user to define the magento ID of a category on website and use it to
import category tree.
Also fix a wrong string for a button in XML.
Merge pull request #94 from shalabhaggarwal/feature/issue-89

Import category tree based on ID specified on website #2096 fixes #89