Fix issue #97 and don't rely on keybindings

The first commit fixes the SparkupNext() function so it doesn't depend on default keymappings. As a Dvorak keyboard user with non-standard movement keys, it was important for me to use Sparkup the way it was supposed to work. The change is more verbose (and probably not as elegant as it could be), but it moves the cursor to the right as intended and doesn't depend on the movement keys being hjkl.

The second commit simply omits `cellspacing` from the built-in or required attributes in a `<table>` element, since the HTML standard doesn't require it. It fixes Issue #97.

I enjoy this plugin and hope my changes are acceptable.