change .h to add setBeta and get Quaternion

discussion from : https://forums.adafruit.com/viewtopic.php?f=19&t=126734&p=632174&hilit=nxp
and arduino-libraries#21

Author: utamadonny

src/MadgwickAHRS.h

@@ -41,6 +41,7 @@ class Madgwick{
 public:
     Madgwick(void);
     void begin(float sampleFrequency) { invSampleFreq = 1.0f / sampleFrequency; }
+    void setBeta( float newBeta ) { beta = newBeta; }
     void update(float gx, float gy, float gz, float ax, float ay, float az, float mx, float my, float mz);
     void updateIMU(float gx, float gy, float gz, float ax, float ay, float az);
     //float getPitch(){return atan2f(2.0f * q2 * q3 - 2.0f * q0 * q1, 2.0f * q0 * q0 + 2.0f * q3 * q3 - 1.0f);};