Important fixes for VAT-included prices [2-3-stable]

This PR enable paypal payments for stores with VAT-included prices (very common in the EU) and should finally solve #129 and #17.

The first fix was already included in `2-2-stable`, this just adds a regression spec.

The second one it's a kind of workaround because paypal does not accept negative taxes amounts.
But this is very important because on Spree we have negative taxes when a country without VAT buys from a VAT-included store, like in this example:
![screen shot 2014-11-29 at 13 36 31](https://cloud.githubusercontent.com/assets/557414/5234461/390d5354-77cd-11e4-9cbe-c3e56f61f186.png)

This is also including all the commits from https://github.com/spree-contrib/better_spree_paypal_express/pull/138 to have green specs.