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 against that repo.

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 the author and commitor by email and through the
travis-ci web UI.

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>