SessionState refactoring; improve token renewal and cookie refresh

* New SessionState to consolidate email, access token and refresh token
* split ServeHttp into individual methods
* log on session renewal
* log on access token refresh
* refactor cookie encription/decription and session state serialization

providers/provider_default_test.go

@@ -0,0 +1,17 @@
+package providers
+
+import (
+	"testing"
+	"time"
+
+	"github.com/bmizerany/assert"
+)
+
+func TestRefresh(t *testing.T) {
+	p := &ProviderData{}
+	refreshed, err := p.RefreshSessionIfNeeded(&SessionState{
+		ExpiresOn: time.Now().Add(time.Duration(-11) * time.Minute),
+	})
+	assert.Equal(t, false, refreshed)
+	assert.Equal(t, nil, err)
+}