Get logs for a contract (#99)

* Add ability to fetch logs for a contract and a block

* Test contract related code against Infura, so can run on Travis

* Add godo task for getLogs

pkg/core/blockchain.go

@@ -10,4 +10,5 @@ type Blockchain interface {
 	StopListening()
 	GetAttributes(contract Contract) (ContractAttributes, error)
 	GetAttribute(contract Contract, attributeName string, blockNumber *big.Int) (interface{}, error)
+	GetLogs(contract Contract, blockNumber *big.Int) ([]Log, error)
 }

pkg/core/log.go

@@ -0,0 +1,9 @@
+package core
+
+type Log struct {
+	BlockNumber int64
+	TxHash      string
+	Address     string
+	Topics      []string
+	Data        string
+}