Add ERC20 token watcher example

- starting with the totalSupply function
- sets contract config on transformer by passing it into the transformer
initializer
- handles block records with the same number for different nodes for both creating token_supply records, and finding missing blocks

examples/erc20_watcher/every_block/fetcher.go

@@ -0,0 +1,70 @@
+// Copyright 2018 Vulcanize
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package every_block
+
+import (
+	"fmt"
+	"github.com/vulcanize/vulcanizedb/pkg/core"
+	"log"
+	"math/big"
+)
+
+type ERC20FetcherInterface interface {
+	FetchSupplyOf(contractAbi string, contractAddress string, blockNumber int64) (big.Int, error)
+	GetBlockchain() core.Blockchain
+}
+
+func NewFetcher(blockchain core.Blockchain) Fetcher {
+	return Fetcher{
+		Blockchain: blockchain,
+	}
+}
+
+type Fetcher struct {
+	Blockchain      core.Blockchain
+	ContractAbi     string
+	ContractAddress string
+}
+
+type fetcherError struct {
+	err         string
+	fetchMethod string
+}
+
+func (fe *fetcherError) Error() string {
+	return fmt.Sprintf("Error fetching %s: %s", fe.fetchMethod, fe.err)
+}
+
+func newFetcherError(err error, fetchMethod string) *fetcherError {
+	e := fetcherError{err.Error(), fetchMethod}
+	log.Println(e.Error())
+	return &e
+}
+
+func (f Fetcher) FetchSupplyOf(contractAbi string, contractAddress string, blockNumber int64) (big.Int, error) {
+	method := "totalSupply"
+	var result = new(big.Int)
+	err := f.Blockchain.FetchContractData(contractAbi, contractAddress, method, nil, &result, blockNumber)
+
+	if err != nil {
+		return *result, newFetcherError(err, method)
+	}
+
+	return *result, nil
+}
+
+func (f Fetcher) GetBlockchain() core.Blockchain {
+	return f.Blockchain
+}