(Feature) finalize immediately with the last vote

[vbaranov]

Relates to poanetwork/RFC#8

[vbaranov]

@varasev were you planning to include this feature request to refactored contracts?

[varasev]

@vbaranov I wasn't, because this is a separate task, but if it's necessary, I'll give this task higher priority and include it in the current set of tasks that should be implemented before hard-fork.

[vbaranov]

@varasev no, this is a lower priority than block reward and additional emission

[varasev]

This was implemented and covered by tests in #113

I didn't include a calling of finalization function inside vote method because there can be a blocking (in case of any errors) if _finalizeBallot reverts, so the last validator won't be able to vote in this case and will be confused.

Therefore I suggest this implementation that assumes any validator should call finalize method separately by the same principle that works now.

These changes cover poanetwork/RFC#8.