First working version of PPLNS payouts

* Based PPLNS on Prop Payout script
* Using defaults from prop payout, no class adjustments
* Added more methods required for PPLNS
* Added block methods for dynamic payout calculations
* Added PPLNS Sidebar that also displays the PPLNS Target
 * Shares beyond this target will not be included in payouts
 * Shares missing to this target will be added from archives
* Enabled archiving by default for PPLNS
* Added configuration options for PPLNS
 * Documented the usage for PPLNS, defaults are sane
* Added pplns_payout to run-crons

Addresses #143 and if accepted will fix it
Abort payout process if missing share_id

As discussed in #392 we should abort any payouts for rounds until the
issue is fixed.