Changed redraw-mechanism to avoid memory leaks due to spin-handler.

Hi Todd,

ProgressWheel is a great library, but one problem which I observed was, you have implemented a Spin-Handler which keeps posting messages for redraw even though the Activity/Fragment is closed, To fix this issue I checked into android's native progress-bar implementation, and I came across very interesting way of redrawing the view after some delay,please refer link below.
http://developer.android.com/reference/android/view/View.html#postInvalidateDelayed(long)

It will be great improvement to ProgressWheel's performance, Please accept my pull-request.