Simplest possible staking system

[futurechimp]

We need to be able to keep a list of mixnodes which are active, and another list of standby nodes. Standby mixnodes could be active but aren't mixing anything right now in order to keep traffic concentrated.

We'll order the list of all mixnodes by stake, and select the top X nodes in order to expand / shrink capacity.

For the moment, we'll simply implement stake as an integer that can be set by anybody. Later, this will change to a more sophisticated system.

[futurechimp]

We're going to call this reputation for the moment so that nobody gets confused between what is effectively a simple, throwaway reputation score and what will later be "real" staking with coins.

[futurechimp]

We have integer reputation now, and will move to token staking on mixnodes in 0.10. I think we can count this one as done now.