Added functions to turn on and turn off the display (#11)

* added functions to turn on and turn off teh display

* Fix comments for on/off

Author: David-OConnor

src/display.rs

@@ -350,6 +350,16 @@ where
     pub fn rotation(&self) -> DisplayRotation {
         self.display_rotation
     }
+
+    /// Turn the display on (eg exiting sleep mode)
+    pub fn turn_on(&mut self) -> Result<(), Error<CommE, PinE>> {
+        Command::DisplayOn(true).send(&mut self.spi, &mut self.dc)
+    }
+
+    /// Turn the display off (enter sleep mode)
+    pub fn turn_off(&mut self) -> Result<(), Error<CommE, PinE>> {
+        Command::DisplayOn(false).send(&mut self.spi, &mut self.dc)
+    }
 }
 
 #[cfg(feature = "graphics")]