Documentation and switch to Exporter

This patch includes:

- Documentation improvements.
- use Exporter in Daiku.pm instead of manually writing import().

When my build script gets very long, I often want to use Daiku without importing their symbols (or at least I want to import them explicitly).

I thought Daiku implemented its own import() because it created a Daiku::Registry object for each importing package. In this patched version, Daiku keeps Registry objects in `%engine_for` hash, so that we can just use Exporter.