Added originalImage that will persist as the image under the strokes.

[gohjohn]

This feature should be a solution to issue #34

Use setOriginalImage to set an image that will be placed under all strokes.

The image will persist undos and clears and redraws.

To remove the image, call setOriginalImage with a nil param

[dehli]

Thanks for the PR @gohjohn! Since TouchDrawView inherits from UIImageView, it already has an image property. I think we can probably solve this only using that instead of introducing another image variable. I'll mess around with getting it working (and if it's not possible I will merge this PR). Cheers! 👍

[gohjohn]

Cheers! 👍

[dehli]

Hey @gohjohn, sorry it took so long for me to get to this! I submitted another PR (#38) where I solved the problem a little differently. If you get a chance to look through it/test it out, let me know your thoughts!

[dehli]

Closing this PR as it's been implemented slightly differently with becd1dc. Let me know if there's any issues with it!