last rank isn't assigned

Hi,

The attached patch fixes a bug I've encountered, where the last rank in the list isn't assigned, an empty string is assigned instead. I also added a new feature, you can specify ranks without post counts in the end of the list, and then assign them manually, I use them on my site for "Admin" and "Mod", these ranks are fixed and not related to post count.

I had to rewrite the loop that chooses the rank, tested the code on my site and it seems to work fine.

Thanks.