Add documentation, sanitise API

Comprehensive API has not yet been added, but I thought I'd push this
once to have at least something.

Changes include:
* A user manual: explains how to use modules (compilation and runtime)
  and how to use X resources to configure karuibar.
* A programmer's manual: how to write a module for karuibar. The API
  will follow soon.
* Sanitised X resources variables. If there is anybody out there using
  karuibar, please update your configuration.
* Clean up commented-out code snippets in time module.