3rd problem added to the homework.

[Subbarao Kambhampati <rao@parichaalak.eas.asu.edu>]

I added a problem on vector space ranking and relevance feedback to
Homework 1. You will notice that I am mostly asking you to verify the
results I had already showed you in the class. 

I also modified qn 2 so it just asks for the Precision/recall curve
rather than asking for specific interpolated values (since people were 
having trouble deciding which interpolation scheme to use). 

Rao