A6 A7 gateway (GSM GPRS) (#195)

* init 2G gateway with SMS sending and receiving

* A6lib add

* add 2Ggateway call from setup and loop

* add schematic and image

lib/A6lib/README.md

@@ -0,0 +1,65 @@
+# A6lib
+
+An ESP8266/Arduino library for communicating with the AI-Thinker A6 GSM module.
+It will probably also work with other GSM modules that use the AT command set,
+like the SIM900.
+
+## Details
+
+This library is an adaptation of
+[this sample module](https://github.com/SensorsIot/A6-GSM-Module). It is still
+very incomplete, but at least it works somewhat. Please feel free to issue pull
+requests to improve it.
+
+## Usage
+
+If you have the A6 breakout board, connect the Tx and Rx pins to your ESP8266.
+The example assumes the TX pin is connected to D5 and the RX to D6. A6lib can
+power-cycle the module if you connect a MOSFET to a pin and control the A6's
+power supply with it.
+
+For a sample circuit that uses this library, have a look at [the A6/ESP8266
+breakout board](https://gitlab.com/stavros/A6-ESP8266-breakout/) I designed.
+
+The A6's PWR pin should be permanently connected to Vcc (if you think that's
+wrong or know a better way, please open an issue).
+
+The code looks something like this:
+
+~~~
+// Instantiate the class with Tx, Rx (remember to swap them when connecting to
+the A6, i.e. connect the A6's Rx pin to D6).
+A6lib A6c(D6, D5);
+
+// Initialize the modem, rebooting it if it fails to become ready.
+do {
+    // Power-cycle the module to reset it.
+    A6c.powerCycle(D0);
+} while (A6c.blockUntilReady(9600) != A6_OK);
+
+// Start and place a call.
+A6c.dial("1234567890");
+delay(8000);
+
+A6c.hangUp();
+delay(8000);
+
+A6c.redial();
+delay(8000);
+
+// Send a message.
+A6c.sendSMS("+1234567890", "Hello there!");
+
+// Get an SMS message from memory.
+SMSmessage sms = A6c.readSMS(3);
+
+// Delete an SMS message.
+A6c.deleteSMS(3);
+
+callInfo cinfo = A6c.checkCallStatus();
+// This will be the calling number, "1234567890".
+cinfo.number;
+~~~
+
+This library doesn't currently include any code to connect to the internet, but
+a PR adding that would be very welcome.