expand attack discussion

It seems that many people bring up reactive attacks when they first see the specification. I think expanding it would help to convey the strong resistance that Cash Accounts have. I tried to take the explanation step by step with more explicit calculations. I also mentioned some math that remains to be done.

For what it is worth, I think the basic calculation will be d = log(1+r+a) where:

• d = average expected collision digits
• r = original registrations
• a = attack registrations

However, that's not exactly it because it changes depending on the balance between r and a.

It would be easy for a statistics person to come up with the expected distribution D of collision digits given r and a.

Todo:

• replace abbreviations with full names
• include cost of defense vs. cost of attack
• clarify system-wide attack