Ally value decay function

Ally price should drop by a set percentage each day. If you express it as a formula, P*e^(-10.95*faction_of_year_since_purchase) shouldn't require any more ompf from the servers, as it could be done client side and only verified by the server. This should cause the value for a certain skillset to stabilize. No longer would someone with three zeros get stuck at a million gold.. unless people actually valued buffer allies that highly. This would also cause unpopular allies to turn a loss, but that should also be fine.

The above is 3% day on day. Adjustments might be desirable.

On the same note, I'd like to be able to finger the parameters by minimum values. Sometimes you get 20 people in a row who can't manage swamps.