Getting the difference of the value of two rows which have the same value in another row

I have the following table:

(table from a previous query)
count(method)    top_url                          Tor

4                http://ift.tt/1beNwMS      Tor
4                http://ift.tt/1beNwMS      NoTor
5                http://thebay.ca                 Tor
119              http://thebay.ca                 NoTor
10               http://test.com                  Tor
5                http://test.com                  NoTor

I would like to get the difference between each rows from the same top_url. And if possible, I would like to get the absolute value (remove the minus sign if there is one).

The output of the previous table would be:

Diff     top_url

0        http://ift.tt/1beNwMS
114      http://thebay.ca
5        http://test.com