Add support for relative geometry.

Currently, supporting a single dzen configuration across multiple computers can be a pain because the computers might have different display resolutions. This patch adds support for specifying the size and position of the dzen window relative to the resolution by specifying a percentage (e.g., dzen -w 50% will start a dzen window taking up half of the screen width-wise).

(Resubmitted because the last pull request was a mess.)