Add travis configuration.

This adds a configuration for travis, a continuous integration server for all projects on GitHub. I've verified that this works with current OpenChange. You can see the current results here: https://travis-ci.org/jelmer/openchange

When merging this, we'll also have to enable travis for the OpenChange repository. You should be able to do this from the travis-ci.org website (I can't since I don't have admin privileges).

Note that this currently only checks that openchange builds, it does not run "make test" yet since that is broken at the moment.