Added basic tests and minor tweaking

This is the best Instapaper library in Python that I’ve found, so I use it regularly, but it doesn’t support the Full API (subscriber-only API calls). I’d like to extend the library to support those methods, and hopefully merge them back into the original tree.

Before I dive in to start changing things around, I wrote a couple of (very basic) tests, so that I can avoid breaking something. I think they might be useful to other people working on the library, so I thought a pull request was in order. I’ll add new tests as I need them, and make appropriate pull requests.

I’ve also made a few minor tweaks to `instapaperlib.py`, but this is mostly cosmetic stuff.