gpio.hpp: Add C++ wrapper around Gpio

* maa_gpio_context bcomes an opaque pointer
* C++ wrapper class Gpio created
* swig now uses C++ wrapper Gpio to generate API

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/isr_pin6.c

@@ -36,7 +36,7 @@ void interrupt (void) {
 int main ()
 {
     maa_init();
-    maa_gpio_context* x;
+    maa_gpio_context x;
 
     x = maa_gpio_init(6);
     if (x == NULL) {