Define common display interface (#31)

* issue #27 definition of Displayer interface

displayer.go

@@ -0,0 +1,14 @@
+package drivers
+
+import "image/color"
+
+type Displayer interface {
+	// Size returns the current size of the display.
+	Size() (x, y int16)
+
+	// SetPizel modifies the internal buffer.
+	SetPixel(x, y int16, c color.RGBA)
+
+	// Display sends the buffer (if any) to the screen.
+	Display() error
+}