Pro-rating memberships, take 2

[Spark]

After discussing how this system works with the creator of the software package, it would appear that we have two choices:

1. We can have the software automatically pro-rate upgrades between subscription levels
2. We can have the software automatically pro-rate downgrades between subscription levels

We can't do both. Since the vast majority of upgrades are from Basic to Gold, I've set it up that way in the system - if you upgrade from a lower level, to a higher level, the previous subscription is cancelled and prorated onto the end of the new one.

I believe I have the bugs worked out, but if there are any issues, please let me know.