Tag release 1.31.

There are no functional changes in this release. Other than some documentation
cleanup, the main changes are the addition of a Travis CI configuration, and
some fixes to the Makefile.

Tag release 1.30.

Improve test automation; no functional changes to the library.

Tag release 1.29.

Documentation Edition. The main improvement here is that the API documentation
is now unified in the header files, and doc.md is now generated from a template
that includes the text from the headers. The automation for this is still
unsatisfactory, but it is better than it was.

Tag release 1.28.

Tag release 1.27.

Tag release 1.26.

Tag release 1.25.

Tag release 1.24.

Bump revision to 1.23.

Bump the revision number to 1.21