Adds support for Grove Magnetic Switch Sensor

- http://wiki.seeed.cc/Grove-Magnetic_Switch/

Signed-off-by: Warren Fernandes <warren.f.fernandes@gmail.com>

drivers/gpio/README.md

@@ -26,6 +26,7 @@ Gobot has a extensible system for connecting to hardware devices. The following
   - Grove Rotary Dial
   - Grove Relay
   - Grove Temperature Sensor
+  - Grove Magnetic Switch Sensor
   - LED
   - Makey Button
   - Motor

drivers/gpio/grove_drivers.go

@@ -174,3 +174,20 @@ func NewGroveTouchDriver(a DigitalReader, pin string, v ...time.Duration) *Grove
 		ButtonDriver: NewButtonDriver(a, pin, v...),
 	}
 }
+
+// GroveMagneticSwitchDriver represent a magnetic
+// switch sensor with a Grove connector
+type GroveMagneticSwitchDriver struct {
+	*ButtonDriver
+}
+
+// NewGroveMagneticSwitchDriver returns a new GroveMagneticSwitchDriver with a polling interval of
+// 10 Milliseconds given a DigitalReader, name and pin.
+//
+// Optionally accepts:
+//  time.Duration: Interval at which the ButtonDriver is polled for new information
+func NewGroveMagneticSwitchDriver(a DigitalReader, pin string, v ...time.Duration) *GroveMagneticSwitchDriver {
+	return &GroveMagneticSwitchDriver{
+		ButtonDriver: NewButtonDriver(a, pin, v...),
+	}
+}

drivers/gpio/grove_drivers_test.go

@@ -13,3 +13,4 @@ var _ gobot.Driver = (*GrovePiezoVibrationSensorDriver)(nil)
 var _ gobot.Driver = (*GroveLedDriver)(nil)
 var _ gobot.Driver = (*GroveRotaryDriver)(nil)
 var _ gobot.Driver = (*GroveRelayDriver)(nil)
+var _ gobot.Driver = (*GroveMagneticSwitchDriver)(nil)