Tracks touches better

[nicholasnge]

added a private boolean currentlyDown;

added a few lines in onTouchEvent.

• sets currentlyDown to true when MotionEvent.ACTION_DOWN,
• sets to false when MotionEvent.ACTION_UP;

tracks touch movement as long as currentlyDown is true. seems to be more intuitive touch movement.

PS. this is my first pull request ever, so apologise in advance if i misunderstand how this process works. Thanks, great library!