Update ProgressWheel.java

Added a public boolean to test if the wheel is currently spinning. Use with .isSpinning()

Added because I found that with multiple calls to .spin() the speed of the spinning on the wheel would increase until it crashes. 

*Note* I imagine this could be included in the .spin() method(? im not sure thats the right term) but I did not want to accidentally break functionality. I am still new to this.
Merge pull request #20 from the4ndy/master

Update ProgressWheel.java