aio.c: added normalized read returning a float in a 0.0-1.0 range

Closes #32.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/aio.hpp

@@ -65,6 +65,14 @@ class Aio {
         int read() {
             return mraa_aio_read(m_aio);
         }
+        /**
+         * Read a value from the AIO pin and return it as a normalized float.
+         *
+         * @returns The current input voltage as a normalized float (0.0f-1.0f)
+         */
+        float readFloat() {
+            return mraa_aio_read_float(m_aio);
+        }
         /**
          * Set the bit value which mraa will shift the raw reading
          * from the ADC to. I.e. 10bits