Adding driver for four-wire resistive touchscreen (#118)

* resistive: Adding driver for four-wire resistive touchscreen, as used on the Adafruit PyPortal.

touch/point.go

@@ -0,0 +1,17 @@
+package touch
+
+// Pointer is a device that is capable of reading a single touch point
+type Pointer interface {
+	ReadTouchPoint() Point
+}
+
+// Point represents the result of reading a single touch point from a screen.
+// X and Y are the horizontal and vertical coordinates of the touch, while Z
+// represents the touch pressure.  In general, client code will want to inspect
+// the value of Z to see if it is above some threshold to determine if a touch
+// is detected at all.
+type Point struct {
+	X int
+	Y int
+	Z int
+}