Disable gateway when user has insufficient points

[JDGrimes]

In the same vein as #36, I wonder if we should disable the gateway when a user has insufficient points to make the purchase. I discovered that the gateways actually have a max_amount property that is automatically taken into account when checking the availability of the gateway to pay for the current order.

[JDGrimes]

We already show an error to the user after they submit the order if they have insufficient points to pay. However, it just seems like we could save them the hassle of that by telling them up front that they can't pay.

Now that users can pay with one of several points types (if the admin allows it), we have to check each points type to see if they have enough. Really, I guess it also makes sense to only show those points types in the dropdown for the user to choose from that they actually have enough of to pay with.