build: Add travis continous integration support

This enables the use of travis-ci via github. Linking any ovs
github repo to travis-ci [0] will automatically lead to a build
and testsuite run being triggered for both commits and pull
requests against the repo.

The introduction of any warnings will cause the build to fail as
both gcc and clang are invoked with -Werror. Failures are reported
to the author and commiter by email, through the travis-ci web UI,
and

The following builds are issued:

For each { gcc + sparse, clang }
  * --disable-ssl
  * --with-linux=linux-3.14.7
  * --with-dpdk=1.7.0

The testsuite is run only in the linux-3.14.7 case to keep the
total build time to an acceptable level.

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