Allow user to save canvas as png.

Manifest: Included write in manifest for filewriter functions.
saveFile: Convert canvas URI to binary/blob so user can save as .png.
main.html: Included save button.

R=mgiuca@chromium.org

Review URL: https://codereview.appspot.com/36870043

Patch from Davina Adisusila <dadisusila@google.com>.