ImageThreshold.js and ImageSelect.js bugfixes

[ccpandhare]

Bugfixes:

ImageSelect.js:
Changed non-existant options.el.html(image) to el.html(image) . This fixes Issue #2

ImageThreshold.js:
The Problem: Sometimes, The module "Image Threshold" was being called before the image loaded. This means the module worked sometimes, and didn't work otherwise. (Crosscheck by repeatedly reloading the /index.html page).
This fixes Issue #2

[jywarren]

I'd love to merge this -- I think we really need more and better tests though so I'm interested in merging my other pr first once I get it passing. What are your thoughts?

[ccpandhare]

Yes you're right. We do need better tests.
But my bugfixes aren't really big, They can be merged at any stage as they don't really interfere with the ones in your PR.
I don't think it would make a difference if they are merged before or after, if at all they are to be.

Do you think so?

[jywarren]

OK, i see you're correct -- ok, merging now, then! Do check out my expanded tests in #1. Still figuring out how to get everything running headlessly, in nodejs.

[jywarren]

I still got:

Uncaught TypeError: Cannot read property 'src' of undefined
    at PixelManipulation (image-sequencer.js:184462)

on this line: https://github.com/publiclab/image-sequencer/blob/master/src/modules/PixelManipulation.js#L17

when adding a new step via the HTML ui. But then when I drop a new image into the image-select UI, all steps run correctly. Opening a new issue for this. Thanks!!