Rewards and utility tracker calculations

[miquelcabot]

• Write calculations for calculateUtility() function on utility tracker smart contracts (ValidatorUtilityTracker, WorkerUtilityTracker, OracleUtilityTracker) taking into consideration all factors of each actor.
• Write calculations for getRewards() function on the EpochRewards smart contract taking into consideration the staked amount and utility of each actor
• Write tests to evaluate calculations precision

[Luka-Loncar]

Blocked by #329 and masa-finance/roadmap#8

[H34D]

@Luka-Loncar is this still blocked by masa-finance/roadmap#8 ? and why?