Make sure we use at least one worker when building HTML
Merge pull request #83 from mook/html-at-least-one-worker

Make sure we use at least one worker when building HTML.

When there are more workers than files, we can end up with a step of 0. xrange really doesn't like that, and throws an exception. This makes sure that it gets at least 1, so that it just iterates over an empty range instead.