Add lint checks: gofmt, goimports, golint

- gofmt - Gofmt checks whether code was gofmt-ed. By default this tool runs
  with -s option to check for code simplification
- goimports - Goimports does everything that gofmt does. Additionally it checks
  unused imports
- golint - Golint differs from gofmt. Gofmt reformats Go source code, whereas
  golint prints out style mistakes
    - checks the uncommented exported functions & types

Update the code to fix the lint checks.

common/l1tx.go

@@ -25,6 +25,7 @@ type L1Tx struct {
 	Type            TxType             `meddler:"tx_type"`
 }
 
+// Tx returns a *Tx from the L1Tx
 func (tx *L1Tx) Tx() *Tx {
 	return &Tx{
 		TxID:    tx.TxID,