Generate shorter tokens

I enjoyed the conversation in #262 so much that I decided to make our tokens less secure.

I don't really want to make big assertions about cryptography, but this has reduced the key space from 10^154 to 10^114, so I think we're probably still ok.

I also didn't want to be *the guy who took it all the way down to 20 characters*, so I've gone for 64 to start with.