New filter: arithop - arithmetic operations on fields

Hi. 

I was looking for a filter that allows to combine two fields into a target field by applying an arithmetic operation. Unfortunately there was none so I created one. 

Really, I'm not a ruby developer - in fact this is the first time ever I've written ruby. 
Please take a look at the code and review it. 

There's a TODO tag that explains an improvement. Because I lack the ruby knowledge (and time) I'm hope someone else can fix this. 

Also I've not written any tests. 

Cheers, 
Fabio

PS: ruby is fun!