Issue 75 : compute tax for whole order coupon

I think I got this one, for issue #75.

Found one additional bug when hunting the original one down : [fix on this commit](https://github.com/abulte/GoCart/commit/17c19ae090e6aa987a7588052aef20bbc1d19e70) 

I made the (essential) assumption that you can cumulate a whole order coupon with a product coupon.

You can see a result of my tests here : http://cl.ly/image/1J1W3M1b222L . Tested with : 
- both taxable and non taxable products
- whole order coupon (5%)
- product coupon on the taxable one (5%)

Hope this helps.

(I messed up in my commit messages, sorry. It's issue #75, not #71)