Add I2S class support (#7874)

Fixes #427

Adds a basic I2S class based off of the Arduino-SAMD core.  The raw
i2s_xxx functions are still a better  way to use I2S due to their
flexibility, but this will allow basic Arduino sketches to work.

libraries/I2S/examples/InputSerialPlotter/InputSerialPlotter.ino

@@ -0,0 +1,36 @@
+/*
+  This example reads audio data from an Invensense's ICS43432 I2S microphone
+  breakout board, and prints out the samples to the Serial console. The
+  Serial Plotter built into the Arduino IDE can be used to plot the audio
+  data (Tools -> Serial Plotter)
+  created 17 November 2016
+  by Sandeep Mistry
+*/
+
+#include <I2S.h>
+
+void setup() {
+  // Open serial communications and wait for port to open:
+  // A baud rate of 115200 is used instead of 9600 for a faster data rate
+  // on non-native USB ports
+  Serial.begin(115200);
+  while (!Serial) {
+    ; // wait for serial port to connect. Needed for native USB port only
+  }
+
+  // start I2S at 8 kHz with 24-bits per sample
+  if (!I2S.begin(I2S_PHILIPS_MODE, 8000, 24)) {
+    Serial.println("Failed to initialize I2S!");
+    while (1); // do nothing
+  }
+}
+
+void loop() {
+  // read a sample
+  int sample = I2S.read();
+
+  if (sample) {
+    // if it's non-zero print value to serial
+    Serial.println(sample);
+  }
+}

libraries/I2S/examples/SimpleTone/SimpleTone.ino

@@ -0,0 +1,46 @@
+/*
+  This example generates a square wave based tone at a specified frequency
+  and sample rate. Then outputs the data using the I2S interface to a
+  MAX08357 I2S Amp Breakout board.
+
+  created 17 November 2016
+  by Sandeep Mistry
+  modified for ESP8266 by Earle F. Philhower, III <earlephilhower@yahoo.com>
+*/
+
+#include <I2S.h>
+
+const int frequency = 440; // frequency of square wave in Hz
+const int amplitude = 500; // amplitude of square wave
+const int sampleRate = 8000; // sample rate in Hz
+
+const int halfWavelength = (sampleRate / frequency); // half wavelength of square wave
+
+short sample = amplitude; // current sample value
+int count = 0;
+
+void setup() {
+  Serial.begin(115200);
+  Serial.println("I2S simple tone");
+
+  // start I2S at the sample rate with 16-bits per sample
+  if (!I2S.begin(I2S_PHILIPS_MODE, sampleRate, 16)) {
+    Serial.println("Failed to initialize I2S!");
+    while (1); // do nothing
+  }
+}
+
+void loop() {
+  if (count % halfWavelength == 0) {
+    // invert the sample every half wavelength count multiple to generate square wave
+    sample = -1 * sample;
+  }
+
+  // write the same sample twice, once for left and once for the right channel
+  I2S.write(sample);
+  I2S.write(sample);
+
+  // increment the counter for the next sample
+  count++;
+}
+