setNeedsDisplay

Hi,

this comment on your blog mentions difficulties to get NISignatureView working:
http://www.nearinfinity.com/blogs/jason_harwig/2012/11/06/capture-a-signature-on-ios.html#comment-772306471

I found that I needed some calls to setNeedsDisplay to make it work.
This pull request also adds a method to get the signature as a UIImage.