build: Add travis continous integration support

This enables the use of travis-ci via github. Linking any ovs
github repo to travis-ci will automatically lead to a build and
testsuite run being triggered for both new commits and for pull
requests.

The build is run with -Werror for both gcc and clang so that the
introduction of a new warning will cause a build to fail. Failures
are reported to both the author and the committer via email.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>