Merge branch 'travis-ci'

Merge branch 'travis-ci'

Adds a config file and build script for Travis CI. Makes the unit tests
buildable with Clang, and test vectors are now actually verified when
the unit tests are executed.

Also adds options to run only selected test suites and to increase the debug
level during unit tests.

The --enable/disable configure options have been reordered and grouped, and
an option to enable all the features has been added (plus an option to
select a specific printf-hook implementation).