Tweak whitespace in a lot of areas for readability and make style tweaks

This PR just tweaks whitespace all over and changes a few variable names to make more sense or fit Ruby style a bit better.  I'm not sold on using dashes on the attributes keys on `Document`, but I don't know if that's a Python/Asciidoc established precedent or not so I just followed it. :)

Feel free to revert any of these you think are necessary names for some reason; I didn't know if you intentionally left things like `assetdir` rather than `asset_dir` or if it's just a style thing.

As for whitespace, I was finding parts of the code hard to read, so I dropped some whitespace in to make it easier for these old eyes to process. ;)  I also tried to put the return value on its own line if it's a bare return (i.e., just returning the last value used in a method).  That's just a style thing I've picked up, so if you hate it, feel free to revert that, too, but I feel like it's much easier to discern what's going on with it like that.