Support clock mocking.

This addresses #15.

Gopkg.lock

@@ -1,6 +1,12 @@
 # This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
 
 
+[[projects]]
+  branch = "master"
+  name = "github.com/benbjohnson/clock"
+  packages = ["."]
+  revision = "7dc76406b6d3c05b5f71a86293cbcf3c4ea03b19"
+
 [[projects]]
   branch = "master"
   name = "rsc.io/qr"
@@ -14,6 +20,6 @@
 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "b705d306da5a78e76b7ff289744770eef56328a4f0d1c615c1a233d056283651"
+  inputs-digest = "6f47334a8fcf2bb48e739e6181089e02bce6cc4d68888be311df3827db96f8ae"
   solver-name = "gps-cdcl"
   solver-version = 1

Gopkg.toml

@@ -32,3 +32,7 @@
 [prune]
   go-tests = true
   unused-packages = true
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/benbjohnson/clock"

totp.go

@@ -11,9 +11,12 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
-	"time"
+
+	"github.com/benbjohnson/clock"
 )
 
+var time clock.Clock
+
 // TOTP represents an RFC 6238 Time-based One-Time Password instance.
 type TOTP struct {
 	*OATH