Provide config option to force SSL in upload url

Fixes url and ssl configurations.

I'm not sure why this doesn't work anymore, nor why it seems to work fine in production machines, but *not* in my development environment. I'm also not sure why this didn't happen before. Maybe an upgrade to Chrome started this.

In any case, I'm not really sure why we don't prefix `data-url` with `https:`. It looks like that was pulled over from [S3DirectUpload](https://github.com/waynehoover/s3_direct_upload) when this was first created. It looks like it has since been updated, however, and I've updated our `url` to match more closely what they do.

This also fixes an untested issue where hard-coding the `url` in the config file didn't actually get use.