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/exittree.go

@@ -6,6 +6,7 @@ import (
 	"github.com/iden3/go-merkletree"
 )
 
+// ExitInfo represents the ExitTree Leaf data
 type ExitInfo struct {
 	AccountIdx  Idx
 	MerkleProof *merkletree.CircomVerifierProof