feat: New Curation Contract out of Staking contract (#169)

* curation: refactor out from staking contract and code improvements

- [x] refactor out of the Staking contract
- [x] add require messages
- [x] clean use of structs
- [x] on tokensReceived, have a special distributor sender that can increase reserves
- [x] receive fees as reserves and validate source
- [x] review natspec

Author: abarmat

contracts/Curation.sol

@@ -3,7 +3,7 @@ pragma experimental ABIEncoderV2;
 
 /*
  * @title Dispute management
- * @notice Provides a way to align the incentives of  participants ensuring that Query Results are trustful.
+ * @notice Provides a way to align the incentives of participants ensuring that Query Results are trustful.
  */
 
 import "./Governed.sol";
@@ -67,14 +67,17 @@ contract DisputeManager is Governed {
     );
     // 1 - mainnet // TODO: EIP-1344 adds support for the Chain ID opcode
     uint256 private constant CHAIN_ID = 1;
-    bytes32 private DOMAIN_SEPARATOR;
 
     // 100% in parts per million
     uint256 private constant MAX_PPM = 1000000;
 
     // 1 basis point (0.01%) is 100 parts per million (PPM)
     uint256 private constant BASIS_PT = 100;
 
+    // -- State --
+
+    bytes32 private DOMAIN_SEPARATOR;
+
     // Disputes created by the Fisherman or other authorized entites
     // @key <bytes32> _disputeID - Hash of readIndex data + disputer data
     mapping(bytes32 => Dispute) public disputes;
@@ -154,7 +157,6 @@ contract DisputeManager is Governed {
         _setArbitrator(_arbitrator);
         token = GraphToken(_token);
         staking = Staking(_staking);
-
         minimumDeposit = _minimumDeposit;
         slashingPercentage = _slashingPercentage;
 
@@ -345,7 +347,6 @@ contract DisputeManager is Governed {
             "Error sending dispute deposit"
         );
 
-        // Log event that we awarded _fisherman _reward in resolving _disputeID
         emit DisputeAccepted(
             _disputeID,
             dispute.subgraphID,
@@ -457,7 +458,6 @@ contract DisputeManager is Governed {
             _deposit
         );
 
-        // Log event that new dispute was created against IndexNode
         emit DisputeCreated(
             disputeID,
             _subgraphID,