Handle headers from POA chain

- Fetching headers from Kovan requires custom type without mixHash/Nonce

pkg/core/header.go

@@ -1,8 +1,31 @@
 package core
 
+import (
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
 type Header struct {
 	Id          int64
 	BlockNumber int64 `db:"block_number"`
 	Hash        string
 	Raw         []byte
 }
+
+type POAHeader struct {
+	ParentHash  common.Hash    `json:"parentHash"       gencodec:"required"`
+	UncleHash   common.Hash    `json:"sha3Uncles"       gencodec:"required"`
+	Coinbase    common.Address `json:"miner"            gencodec:"required"`
+	Root        common.Hash    `json:"stateRoot"        gencodec:"required"`
+	TxHash      common.Hash    `json:"transactionsRoot" gencodec:"required"`
+	ReceiptHash common.Hash    `json:"receiptsRoot"     gencodec:"required"`
+	Bloom       types.Bloom    `json:"logsBloom"        gencodec:"required"`
+	Difficulty  *hexutil.Big   `json:"difficulty"       gencodec:"required"`
+	Number      *hexutil.Big   `json:"number"           gencodec:"required"`
+	GasLimit    hexutil.Uint64 `json:"gasLimit"         gencodec:"required"`
+	GasUsed     hexutil.Uint64 `json:"gasUsed"          gencodec:"required"`
+	Time        *hexutil.Big   `json:"timestamp"        gencodec:"required"`
+	Extra       hexutil.Bytes  `json:"extraData"        gencodec:"required"`
+	Hash        common.Hash    `json:"hash"`
+}

pkg/core/node_info.go

@@ -13,6 +13,10 @@ const (
 	GANACHE
 )
 
+const (
+	KOVAN_NETWORK_ID = 42
+)
+
 type Node struct {
 	GenesisBlock string
 	NetworkID    float64