Expose the Account API (RADIUS users)

* Adds Account CRUD API
* Enahnces `emptyStringInt` with a predicate for dynamically setting
  which value should be interpreted as a blank JSON payload

unifi/json.go

@@ -31,3 +31,11 @@ func (e *emptyStringInt) UnmarshalJSON(b []byte) error {
 	*e = emptyStringInt(i)
 	return nil
 }
+
+func (e *emptyStringInt) MarshalJSON() ([]byte, error) {
+	if e == nil || *e == 0 {
+		return []byte(`""`), nil
+	}
+
+	return []byte(strconv.Itoa(int(*e))), nil
+}