Add `FileUtil` for saving string to file, easy debug
Receive the SMS and extract the code using `String.indexOf`
Fix typo in debug log
Add constant for receive SMS intent
add license to README
Extract the sms code by regex
Add `ReceiveSmsBroadcastReceiver` to `AndroidManifest.xml`
Trigger `submitSmsReservationCode` in `MainActivity` after receiving the
SMS reservation code.
Add related function to `ReserveWorker`
Add `getCommonAjax()` to simplify the repeat coding
Check submit reservation code result.