Sdf download

@joetsoi I've merged ckanext-downloadsdf into this extension, and also added inlining of the CSV files in the ZIP files, and some tests. There was actually a couple of bugs that only show when you run the tests with both plugins enabled. I refactored a little to get the inlining code to fit in. Can you review and merge this? Once merged we should change the packaging to not install ckanext-downloadsdf.

There are a couple of lines not covered in the tests (see coverage report), the first is to do with unnamed resources / resources with the same name in CKAN, we need to generate/enforce unique resource names because we need unique filenames in the ZIP files, I'll work on that separately.

The other uncovered line looks like something from your code for making the zip file, can you add a test to cover this?

Cheers