Add sort_by query param to /signers endpoint

[zone117x]

It's currently sorted by (weight, stacked_amount). I'll add a slot_index property.

We could also add something like a ?sort_by=weight|slot_index|etc query param.

Originally posted by @zone117x in #40

Something like:

GET /v1/cycle/{n}/signers?
   sort_by={slot_index|weight|last_seen|response_time|accepted_percent|voted_percent}
   &order={asc|desc}