Fix the jobLimit of the imageLoader

I totally forgot about the issue #490 and fixed this now in the collections branch. I didn't want to fix it in master, since the structure is different (ImageLoader gets initialized in Drawer there) and so you would have to merge it.

**Update**
I just fixed another issue that has caused the imageLoaderLimit to not work, it wasn't incremented after starting a job.

Fixes #490
We should tie this to the documentation for imageLoaderLimit, something like so:

` * @param {Number} [options.jobLimit] - The number of concurrent image requests. See imageLoaderLimit in {@link OpenSeadragon.Options} for details.`