Adding min and max yHeight/xWidth options for scroll bar handle

This is a feature addition not a bug, so I will not be offended if you don't want to use it.  I found that it was kind of frustrating when navigating across a long list of items to have the thumb of the scrollbar keep shrinking down to only a few pixels.

I updated the readme to include documentation for the options.  I could definitely see adding some defaults for this but I didn't want to break any existing functionality so I did not add them myself.

(Also I had no idea what the part was called - saw it called "thumb" here:  http://my.safaribooksonline.com/book/programming/microsoft-wpf/9780768695519/advanced-scrolling/ch07lev1sec1)