Availability check

Added new functions to DeviceMotion and DeviceOrientation classes.
After the promise resolves you can check on the availability of sensor data.
For device orientation:

    var promise = new FULLTILT.getDeviceOrientation({ 'type': 'world' });
    promise.then(function(deviceOrientation) {
        
        deviceOrientation.isAvailable(); // returns true if alpha, beta and gamma values exist and not null

    });

and for DeviceMotion:

    var promise = new FULLTILT.getDeviceMotion({ 'type': 'world' });
    promise.then(function(deviceMotion) {
        
        deviceMotion.isAvailable(deviceMotion.ACCELERATION); 
        // returns true if acceleration values exist and not null

        deviceMotion.isAvailable(deviceMotion.ACCELERATION_INCLUDING_GRAVITY); 
        // returns true if acceleration including gravity values exist and not null

        deviceMotion.isAvailable(deviceMotion.ROTATION_RATE); 
        // returns true if rotation rate values exist and not null

        deviceMotion.isAvailable(); 
        // returns an object for the availability of the sensor data
        /*
            {
             acceleration:<true or false>,
             accelerationIncludingGravity:<true or false>,
             rotationRate:<true or false>
             }
        */

    });

You can see a sample usage in the data_display.html in the examples.
Let me know what you think.
Cheers!

Typo `accelecation` should be `acceleration`