Added the option 'nounload'

[royhaddad]

If the image data was loaded into an object URL, this option prevents it from
being unloaded once the image is ready.

[royhaddad]

Without this option, in browsers with Files / Blobs, loaded images cannot be manipulated using pixastic (before doing anything, pixastic creates a second <img/>, and copies the src, which is a blob URL. The resulting GET fails because it has already been unloaded).

[blueimp]

Thanks for your contribution.
I've refactored your commit slightly and renamed the option to noRevoke.

[joewood]

I think the code merge now breaks if 'option' is undefined. The line:

   if (oUrl && !options.noRevoke) {
                    loadImage.revokeObjectURL(oUrl);
   }

Needs to test for options being undefined before using it.

[blueimp]

Thanks Joe, your pull request has been merged.