Added support for associations from categories to entities and vice versa.
So, essentially, one can now almost draw diagrams for EER models as given in this book
<a href="http://www.amazon.com/Fundamentals-Database-Systems-Fourth-Elmasri/dp/0321122267">http://www.amazon.com/Fundamentals-Database-Systems-Fourth-Elmasri/dp/0321122267</a>;

Still TODO:
* change properties of categories ( currently I've set "disjoint specialisation" as default)
* xmi representation of all the foobar I did :P
* and SQL code generation for EER too probably. This should be interesting.