Continuous integration, fix minor glitches, ready for release

- Travis CI support (see, e.g., https://travis-ci.org/krlmlr/ProjectTemplate/builds/13663769 for test results of this pull request)
- Dependencies
- Reenable xlsx
- project.info via `makeActiveBinding`, get rid of NOTEs with `R CMD CHECK`

I could gradually take over maintenance of the package. Would you like to invite me as collaborator to your repo?