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/gpio_read6.c

@@ -34,7 +34,7 @@ main(int argc, char **argv)
     fprintf(stdout, "MAA Version: %s\nStarting Read on IO6\n",
             maa_get_version());
 
-    maa_gpio_context* gpio;
+    maa_gpio_context gpio;
 
     gpio = maa_gpio_init(6);