small update to use of sandbox

[tomscytale]

before now the paypal sandbox was used inconsistently

this patch means that the sandbox will be used whenever django-paypal's TEST setting is true. This in turn is equal to the value of (django) settings.PAYPAL_TEST (if it is set) or settings.DEBUG

thanks

[poswald]

This seems to work well. Much better than the two separate methods. Thanks!

[Narsil]

Any news from this pull request ?

[tomscytale]

currently trying to contact dcramer as there are other updates to django-paypal I'd like to make, no success yet