critical bugfix: fixed node range assignment

After deploying the script in my system I was quite impressed to see that cluster-wide repairs that took many hours before were taking only a few minutes. 

In order to investigate this, I manually ran `nodetool repair -par -pr ks cf` in one of the nodes (with token **133971098536869478390145977979120800266**) and got the following token range for that node:
(127605887595351923798765477786913079296,**133971098536869478390145977979120800266**)
The execution took about 6 minutes for that particular column family. 

Then I went and executed `range_repair.py -par -k ks -c cf -s 1 -v -d`, and was surprised to find out that the script considered the following token range for that node: 
(**133971098536869478390145977979120800266**, 134695103572871475120919115441741583695)
The execution took a few seconds, which was quite strange. 

When observing the generated range by the script, and the actual node range (as generated by nodetool), I quickly realized the script was considering a node's range the region between its token and the NEXT node's token, when in reality a node's range is the region between its token and the PREVIOUS node's token.

This PR fixes this behavior. If what I said above is correct (which I believe it is), the script never worked before.

You can confirm that a node's range is comprised by the region of the ring between itself and its predecessor by checking the following cassandra docs:
* http://www.datastax.com/docs/1.1/cluster_architecture/partitioning
* http://www.datastax.com/documentation/cassandra/2.0/cassandra/architecture/architectureDataDistributeHashing_c.html
* http://www.datastax.com/documentation/cassandra/2.0/cassandra/architecture/architectureDataDistributeDistribute_c.html (the same is valid for virtual nodes)