Migration file based on #83

I choosed Rev3, because Rev2 will be about "open invitations" feature.

pkg/database/migrate/201402211147_github_enterprise_support.go

@@ -0,0 +1,23 @@
+package migrate
+
+type Rev3 struct{}
+
+var GitHubEnterpriseSupport = &Rev3{}
+
+func (r *Rev3) Revision() int64 {
+	return 201402211147
+}
+
+func (r *Rev3) Up(op Operation) error {
+	_, err := op.AddColumn("settings", "github_domain VARCHAR(255)")
+	if err {
+		return err
+	}
+	_, err = op.AddColumn("settings", "github_apiurl VARCHAR(255)")
+	return err
+}
+
+func (r *Rev3) Down(op Operation) error {
+	_, err := op.DropColumns("settings", []string{"github_domain", "github_apiurl"})
+	return err
+}