Change optional `qty` argument to get_note_list() to `since` date string

As per this issue: https://github.com/mrtazz/simplenote.vim/issues/38. Since the order of the note index cannot be guaranteed by the API (newest first, oldest first) limiting the quantity of the notes returned was not fulfilling it's original intention: a way to quickly retrieve recent notes and not the full index.

It makes sense to use the `since` parameter provided by the API to do this.

So this commit removes the "limit list" functionality from `get_note_list()` (the optional `qty` argument) and replaces it with an optional `since` date string in YYYY-MM-DD format.

I've removed the tests that I added for the `qty` argument, but I haven't added any new ones for the `since` argument. This is because I'm not sure how to test it. As far as I can tell the tests currently delete and re-create notes each time so I really don't know how you could guarantee a number of notes have been modified since a particular date. Any ideas?

Also, I've not touch the docs directory. I think this is all automatically generated?

Finally, I've also increased the number of notes retrieved  via the API from 20 to the max allowable 100 (as a separate commit, but included in this pull request).